--- 1/draft-ietf-idr-as-migration-03.txt 2015-04-09 07:14:55.773159893 -0700 +++ 2/draft-ietf-idr-as-migration-04.txt 2015-04-09 07:14:55.809160780 -0700 @@ -1,137 +1,131 @@ Internet Engineering Task Force W. George Internet-Draft Time Warner Cable Intended status: Standards Track S. Amante -Expires: April 2, 2015 Apple, Inc. - September 29, 2014 +Expires: October 11, 2015 Apple, Inc. + April 9, 2015 - Autonomous System Migration Features and Their Effects on the BGP + Autonomous System Migration Mechanisms and Their Effects on the BGP AS_PATH Attribute - draft-ietf-idr-as-migration-03 + draft-ietf-idr-as-migration-04 Abstract - This draft discusses some BGP features for ASN migration that, while - commonly used, are not formally part of the BGP4 protocol + This draft discusses some commonly-used BGP mechanisms for ASN + migration that are not formally part of the BGP4 protocol specification and may be vendor-specific in exact implementation. It is necessary to document these de facto standards to ensure that they - are properly supported in future BGP protocol work such as BGPSec. + are properly supported in future BGP protocol work. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on April 2, 2015. + This Internet-Draft will expire on October 11, 2015. Copyright Notice - Copyright (c) 2014 IETF Trust and the persons identified as the + Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Documentation note . . . . . . . . . . . . . . . . . . . 3 - 2. ASN Migration Scenario Overview . . . . . . . . . . . . . . . 4 - 3. External BGP Autonomous System Migration Features . . . . . . 6 - 3.1. Modify Inbound BGP AS_PATH Attribute . . . . . . . . . . 6 + 2. ASN Migration Scenario Overview . . . . . . . . . . . . . . . 3 + 3. External BGP Autonomous System Migration Mechanisms . . . . . 5 + 3.1. Modify Inbound BGP AS_PATH Attribute . . . . . . . . . . 5 3.2. Modify Outbound BGP AS_PATH Attribute . . . . . . . . . . 7 3.3. Implementation . . . . . . . . . . . . . . . . . . . . . 8 - 4. Internal BGP Autonomous System Migration Features . . . . . . 9 - 4.1. Internal BGP Alias . . . . . . . . . . . . . . . . . . . 10 + 4. Internal BGP Autonomous System Migration Mechanisms . . . . . 9 + 4.1. Internal BGP Alias . . . . . . . . . . . . . . . . . . . 9 4.2. Implementation . . . . . . . . . . . . . . . . . . . . . 12 5. Additional Operational Considerations . . . . . . . . . . . . 13 - 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 14 - 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 - 9. Security Considerations . . . . . . . . . . . . . . . . . . . 14 - 10. Appendix: Implementation report . . . . . . . . . . . . . . . 15 - 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 - 11.1. Normative References . . . . . . . . . . . . . . . . . . 15 - 11.2. Informative References . . . . . . . . . . . . . . . . . 15 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 + 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 + 9. Appendix: Implementation report . . . . . . . . . . . . . . . 14 + 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 + 10.1. Normative References . . . . . . . . . . . . . . . . . . 14 + 10.2. Informative References . . . . . . . . . . . . . . . . . 15 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction - This draft discusses some BGP features for ASN migration that, while - commonly used, are not formally part of the BGP4 [RFC4271] protocol - specification and may be vendor-specific in exact implementation. - These features are local to a given BGP Speaker and do not require - negotiation with or cooperation of BGP neighbors. The deployment of - these features do not need to interwork with one another to - accomplish the desired results, so slight variations between existing - vendor implementations exist, and will not necessarily be harmonized - due to this document. However, it is necessary to document these de - facto standards to ensure that new implementations can be successful, - and any future protocol enhancements to BGP that propose to read, - copy, manipulate or compare the AS_PATH attribute can do so without - inhibiting the use of these very widely used ASN migration features. + This draft discusses some commonly-used BGP mechanisms for Autonomous + System Number (ASN) migration that are not formally part of the BGP4 + [RFC4271] protocol specification and may be vendor-specific in exact + implementation. These mechanisms are local to a given BGP Speaker + and do not require negotiation with or cooperation of BGP neighbors. + The deployment of these mechanisms do not need to interwork with one + another to accomplish the desired results, so slight variations + between existing vendor implementations exist, and will not + necessarily be harmonized due to this document. However, it is + necessary to document these de facto standards to ensure that new + implementations can be successful, and any future protocol + enhancements to BGP that propose to read, copy, manipulate or compare + the AS_PATH attribute can do so without inhibiting the use of these + very widely used ASN migration mechanisms. - The migration features discussed here are useful to ISPs and + The migration mechanisms discussed here are useful to ISPs and organizations of all sizes, but it is important to understand the - business need for these features and illustrate why they are so + business need for these mechanisms and illustrate why they are so critical for ISPs' operations. During a merger, acquisition or divestiture involving two organizations it is necessary to seamlessly migrate both internal and external BGP speakers from one ASN to a second ASN. The overall goal in doing so is to simplify operations through consistent configurations across all BGP speakers in the - combined network. In addition, it is common practice in the industry - for ISPs to bill customers based on utilization. ISPs bill customers - based on the 95th percentile of the greater of the traffic sent or - received, over the course of a 1-month period, on the customer's - access circuit. Given that the BGP Path Selection algorithm selects - routes with the shortest AS_PATH attribute, it is critical that the - ISP does not increase AS_PATH length during or after ASN migration - toward downstream transit customers or settlement-free peers, who are - likely sending or receiving traffic from those transit customers. - This would not only result in sudden changes in traffic patterns in - the network, but also substantially decrease utilization driven - revenue at the ISP. + combined network. In addition, given that the BGP Path Selection + algorithm selects routes with the shortest AS_PATH attribute, it is + critical that the ISP does not increase AS_PATH length during or + after ASN migration, because an increased AS_PATH length would likely + result in sudden, undesirable changes in traffic patterns in the + network. By default, the BGP protocol requires an operator to configure a router to use a single remote ASN for the BGP neighbor, and the ASN must match on both ends of the peering in order to successfully negotiate and establish a BGP session. Prior to the existence of - these migration features, it would have required an ISP to coordinate - an ASN change with, in some cases, tens of thousands of customers. - In particular, as each router is migrated to the new ASN, to avoid an - outage due to ASN mismatch, the ISP would have to force all customers - on that router to change their router configurations to use the new - ASN immediately after the ASN change. Thus, it becomes critical to - allow the ISP to make this process a bit more asymmetric, so that it - could seamlessly migrate the ASN within its network(s), but allow the - customers to gradually migrate to the ISP's new ASN at their leisure, - either by coordinating individual reconfigurations, or accepting - sessions using either the old or new ASN to allow for truly + these migration mechanisms, it would have required an ISP to + coordinate an ASN change with, in some cases, tens of thousands of + customers. In particular, as each router is migrated to the new ASN, + to avoid an outage due to ASN mismatch, the ISP would have to force + all customers on that router to change their router configurations to + use the new ASN immediately after the ASN change. Thus, it becomes + critical to allow the ISP to make this process a bit more asymmetric, + so that it could seamlessly migrate the ASN within its network(s), + but allow the customers to gradually migrate to the ISP's new ASN at + their leisure, either by coordinating individual reconfigurations, or + accepting sessions using either the old or new ASN to allow for truly asymmetric migration. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 1.2. Documentation note @@ -196,537 +190,521 @@ The general order of operations, typically carried out in a single maintenance window by the network undergoing ASN migration (ISP B), are as follows. First, ISP B will change the global BGP ASN used by a Provider Edge (PE) router, from ASN 64510 to 64500. At this point, the router will no longer be able to establish eBGP sessions toward the existing Customer Edge (CE) devices that are attached to it and still using AS 64510. Second, since ISP B needs to do this without coordinating the simultaneous change of its ASN with all of its eBGP peers, ISP B will configure two separate, but related ASN migration - features discussed in this document on all eBGP sessions toward all - CE devices. These features enable the router to establish BGP + mechanisms discussed in this document on all eBGP sessions toward all + CE devices. These mechanisms enable the router to establish BGP neighbors using the legacy ASN, modify the AS_PATH attribute received from a CE device when advertising it further, and modify AS_PATH when transmitted toward CE devices to achieve the desired effect of not increasing the length of the AS_PATH. At the conclusion of the ASN migration, the CE devices at the edge of the network are not aware of the fact that their upstream router is now in a new ASN and do not observe any change in the length of the AS_PATH attribute. However, after the changes discussed in this document are put in place by ISP A', there is a change to the contents of the AS_PATH attribute to ensure the AS_PATH is not artificially lengthened while these AS migration parameters are used. In this use case, neither ISP is using BGP Confederations RFC 5065 [RFC5065] internally. - There are multiple implementations with equivalent features deployed - and in use. Some documentation pointers to these implementations, as - well as additional documentation on migration scenarios can be found - in the appendix. The examples cited below use Cisco IOS CLI for ease - of illustration purposes only. - -3. External BGP Autonomous System Migration Features +3. External BGP Autonomous System Migration Mechanisms - The following section addresses features that are specific to - modifying the AS_PATH attribute at the Autonomous System Border - Routers (ASBRs) of an organization, (typically a single Service - Provider). This ensures that external BGP customers/peers are not - forced to make any configuration changes on their CE routers before - or during the exact time the Service Provider wishes to migrate to a - new, permanently retained ASN. Furthermore, these features eliminate - the artificial lengthening of the AS_PATH both transmitted from and - received by the Service Provider that is undergoing AS Migration, - which would have negative implications on path selection by external - networks. + The following section addresses optional capabilities that are + specific to modifying the AS_PATH attribute at the Autonomous System + Border Routers (ASBRs) of an organization, (typically a single + Service Provider). This ensures that external BGP customers/peers + are not forced to make any configuration changes on their CE routers + before or during the exact time the Service Provider wishes to + migrate to a new, permanently retained ASN. Furthermore, these + mechanisms eliminate the artificial lengthening of the AS_PATH both + transmitted from and received by the Service Provider that is + undergoing AS Migration, which would have negative implications on + path selection by external networks. 3.1. Modify Inbound BGP AS_PATH Attribute - The first feature used in the process described above is called - "Local AS". This feature allows the PE router that was formerly in - ISP B to re-establish an eBGP session toward the existing CE devices - using the legacy AS, AS 64510. Ultimately, the CE devices (i.e.: - customer C) are completely unaware that ISP B has reconfigured its - router to participate as a member of a new AS. Within the context of - the former ISP B PE router, the second effect this feature has on - AS_PATH is that, by default, it prepends all received BGP UPDATEs - with the legacy AS of ISP B: AS 64510, while advertising it (Adj-RIB- - Out) to other BGP speakers (A'). Within Loc-RIB on ISP B prior to - the migration, the AS_PATH toward customer C would appear as: 64510, - whereas the same RIB on ISP A' (ISP B routers post-migration) would - contain AS_PATH: 64510 64496. To avoid changes to the AS_PATH - length, a secondary feature "No Prepend" is added to the "Local AS" - configuration toward every eBGP neighbor on PE routers migrating from - ISP B. The "No Prepend" feature causes those routers to not prepend - the legacy AS, AS 64510, when advertising UPDATES received from - customer C. This restores the AS_PATH within ISP A' toward customer - C so that it is just one ASN in length: 64496. + The first instrument used in the process described above is called + "Local AS". This allows the router to supersede the globally + configured ASN in the "My Autonomous System" field of the BGP OPEN + [RFC4271] with a locally defined AS value, usually configured on a + per-neighbor basis. This mechanism allows the PE router that was + formerly in ISP B to establish an eBGP session toward the existing CE + devices using the legacy AS, AS 64510. Ultimately, the CE devices + (i.e.: customer C) are completely unaware that ISP B has reconfigured + its router to participate as a member of a new AS. Within the + context of the former ISP B PE router, the second effect this + specific mechanism has on AS_PATH is that, by default, it prepends + all received BGP UPDATEs with the legacy AS of ISP B: AS 64510, while + advertising it (Adj-RIB-Out) to other BGP speakers (A'). Within the + Loc-RIB on ISP B prior to the migration, the AS_PATH toward customer + C would appear as: 64510, whereas the same RIB on ISP A' (ISP B + routers post-migration) would contain AS_PATH: 64510 64496. + + A second instrument, referred to as "No Prepend Inbound", is enabled + on PE routers migrating from ISP B. The "No Prepend Inbound" + capability causes ISP B's routers to not prepend the legacy AS, AS + 64510, when advertising UPDATES received from customer C. This + restores the AS_PATH within ISP A' toward customer C so that it is + just one ASN in length: 64496. In the direction of CE -> PE (inbound): - 1. 'local-as ': prepends the value to the AS_PATH - when advertising routes received from the CE + 1. "Local AS": Allows the local BGP router to generate a BGP OPEN to + an eBGP neighbor with the old, legacy ASN value in the "My + Autonomous System" field. When this capability is activated, it + also causes the local router to prepend the value to + the AS_PATH when advertising routes received from a CE to iBGP + neighbors inside the Autonomous System. - 2. 'local-as no-prepend': does not prepend value - to the AS_PATH when advertising routes received from the CE + 2. "No Prepend Inbound (of Local AS)": the local BGP router does not + prepend value to the AS_PATH when advertising routes + received from the CE to iBGP neighbors inside the Autonomous + System - PE-B is a PE that was originally in ISP B, and has a customer peer - CE-B. PE-B has had its global configuration ASN changed from AS - 64510 to AS 64500 to make it part of the permanently retained ASN. - This now makes PE-B a member of ISP A'. PE-A is a PE that was + PE-B is a PE that was originally in ISP B, and has a customer eBGP + session to CE-B. PE-B has had its global configuration ASN changed + from AS 64510 to AS 64500 to make it part of the permanently retained + ASN. This now makes PE-B a member of ISP A'. PE-A is a PE that was originally in ISP A, and has a customer peer CE-A. Although its global configuration ASN remains AS 64500, throughout this exercise we also consider PE-A a member of ISP A'. ISP A' ISP A' CE-A <--- PE-A <------------------- PE-B <--- CE-B 64499 New_ASN: 64500 Old_ASN: 64510 64496 New_ASN: 64500 Note: Direction of BGP UPDATE as per the arrows. - Figure 3: Local AS BGP UPDATE Diagram - - The final configuration on PE-B after completing the "Local AS" - portion of the AS migration is as follows: - - router bgp 64500 - neighbor remote-as 64496 - neighbor local-as 64510 no-prepend + Figure 3: Local AS and No Prepend BGP UPDATE Diagram - As a result of the "Local AS No Prepend" configuration, on PE-B, CE-A - will see an AS_PATH of: 64500 64496. CE-A will not receive a BGP - UPDATE containing AS 64510 in the AS_PATH. (If only the "local-as - 64510" feature was configured without the keyword "no-prepend" on PE- - B, then CE-A would see an AS_PATH of: 64496 64510 64500, which - results in an unacceptable lengthening of the AS_PATH). + As a result using both the "Local AS" and "No Prepend Inbound" + capabilities on PE-B, CE-A will see an AS_PATH of: 64500 64496. CE-A + will not receive a BGP UPDATE containing AS 64510 in the AS_PATH. + (If only the "Local AS" mechanism was configured without "No Prepend + Inbound" on PE-B, then CE-A would have seen an AS_PATH of: 64496 + 64510 64500, which results in an unacceptable lengthening of the + AS_PATH). 3.2. Modify Outbound BGP AS_PATH Attribute - The previous feature, "Local AS No Prepend", was designed to modify - the AS_PATH Attribute received by the ISP in updates from CE devices, - when CE devices still have an eBGP session established with the ISPs - legacy AS, (AS64510). In some existing implementations, "Local AS No - Prepend" does not concurrently modify the AS_PATH Attribute for BGP - UPDATEs that are transmitted by the ISP to CE devices. Specifically, - with "Local AS No Prepend" enabled on PE-B, it automatically causes a - lengthening of the AS_PATH in outbound BGP UPDATEs from ISP A' toward - directly attached eBGP speakers, (Customer C in AS 64496). This is - the result of the "Local AS No Prepend" feature automatically - appending the new global configuration ASN, AS64500, after the legacy - ASN, AS64510, in BGP UPDATEs that are transmitted by PE-B to CE-B. - The end result is that customer C, in AS 64496, will receive the - following AS_PATH: 64510 64500 64499. Therefore, if ISP A' takes no - further action, it will cause an unacceptable increase in AS_PATH - length within customer's networks directly attached to ISP A'. + The two aforementioned mechanisms, "Local AS" and "No Prepend + Inbound", only modify the AS_PATH Attribute received by the ISP's + PE's in the course of processing BGP UPDATEs from CE devices when CE + devices still have an eBGP session established with the ISPs legacy + AS, (AS64510). - A second feature was designed to resolve this problem (continuing the - use of Cisco CLI in the examples, it is called "Replace AS" in the - examples below). This feature allows ISP A' to prevent routers - configured with this feature from appending the global configured AS - in outbound BGP UPDATEs toward its customer's networks configured - with the "Local AS" feature. Instead, only the historical (or - legacy) AS will be prepended in the outbound BGP UPDATE toward the - customer's network, restoring the AS_PATH length to what it what was - before AS Migration occurred. + In some existing implementations, "Local AS" and "No Prepend Inbound" + does not concurrently modify the AS_PATH Attribute for BGP UPDATEs + that are transmitted by the ISP's PE's to CE devices. In these + implementations, with "Local AS" and "No Prepend Inbound" used on PE- + B, it automatically causes a lengthening of the AS_PATH in outbound + BGP UPDATEs from ISP A' toward directly attached eBGP speakers, + (Customer C in AS 64496). The externally observed result is that + customer C, in AS 64496, will receive the following AS_PATH: 64510 + 64500 64499. Therefore, if ISP A' takes no further action, it will + cause an unacceptable increase in AS_PATH length within customer's + networks directly attached to ISP A'. + + A tertiary mechanism is used to resolve this problem, referred to as + "Replace Old AS". This capability allows ISP A' to prevent routers + from appending the globally configured ASN in outbound BGP UPDATEs + toward directly attached eBGP neighbors that are using the "Local AS" + mechanism. Instead, only the old (or previously used) AS will be + prepended in the outbound BGP UPDATE toward the customer's network, + restoring the AS_PATH length to what it what was before AS Migration + occurred. To re-use the above diagram, but in the opposite direction, we have: ISP A' ISP A' CE-A ---> PE-A -------------------> PE-B ---> CE-B 64499 New_ASN: 64500 Old_ASN: 64510 64496 New_ASN: 64500 Note: Direction of BGP UPDATE as per the arrows. Figure 4: Replace AS BGP UPDATE Diagram - The final configuration on PE-B after completing the "Replace AS" - portion of the AS migration is as follows: + By default, without the use of "Replace Old AS", CE-B would see an + AS_PATH of: 64510 64500 64499, which is artificially lengthened, + typically by use of the "Local AS" and/or "No Prepend" capabilities + during the course of the ASN Migration. After ISP A' changes PE-B to + use "Replace Old AS", CE-B would receive an AS_PATH of: 64510 64499, + which is the same AS_PATH length pre-AS migration. NOTE: If there + are still routers in the old ASN, it is possible for them to accept + these manipulated routes as if they have not already passed through + their ASN, potentially causing a loop, since BGP's normal loop- + prevention behavior of rejecting routes that include its ASN in the + path will not catch these. Careful filtering between routers + remaining in the old ASN and routers migrated to the new ASN is + necessary to minimize the risk of routing loops. - router bgp 64500 - neighbor remote-as 64496 - neighbor local-as 64510 no-prepend replace-as +3.3. Implementation - By default, without "Replace AS" enabled, CE-B would see an AS_PATH - of: 64510 64500 64499, which is artificially lengthened by the ASN - Migration. After ISP A' changes PE-B to include the "Replace AS" - feature, CE-B would receive an AS_PATH of: 64510 64499, which is the - same AS_PATH length pre-AS migration. + While multiple implementations already exist, the following documents + the expected behavior such that a new implementation of this + mechanism could be done on other platforms. -3.3. Implementation + These mechanisms MUST be configurable on a per-neighbor or per peer- + group basis to allow for maximum flexibility. When the "Local AS" + capability is used, a local ASN will be provided in the configuration + that is different from the globally-configured ASN of the BGP router. + To implement this mechanism, a BGP speaker MUST send BGP OPEN + [RFC4271] (see section 4.2) messages to the configured eBGP peer(s) + using the local ASN configured for this session as the value sent in + "My Autonomous System". The BGP router MUST NOT use the ASN + configured globally within the BGP process as the value sent in "My + Autonomous System" in the OPEN message. This will avoid causing the + eBGP neighbor to unnecessarily generate a BGP OPEN Error message "Bad + Peer AS". This method is typically used to re-establish eBGP + sessions with peers expecting the legacy ASN after a router has been + moved to a new ASN. - While multiple implementations already exist, the following should - document the expected behavior such that a new implementation of this - feature could be done on other platforms. + Implementations MAY support a more flexible model where the eBGP + speaker attempts to open the BGP session using either the ASN + configured as "Local AS" or the globally configured AS as discussed + in BGP Alias (Section 4.2). If the session is successfully + established to the globally configured ASN, then the modifications to + AS_PATH described in this document SHOULD NOT be performed, as they + are unnecessary. The benefit to this more flexible model is that it + allows the remote neighbor to reconfigure to the new ASN without + direct coordination between the ISP and the customer. - These features MUST be configurable on a per-neighbor or per peer- - group basis to allow for maximum flexibility. When this feature set - is invoked, an ASN that is different from the globally-configured ASN - is provided as a part of the command as exemplified above. To - implement this feature, a BGP speaker MUST send BGP OPEN messages to - the configured eBGP peer using the ASN configured for this session as - the value sent in MY ASN. The speaker MUST NOT use the ASN - configured globally within the BGP process as the value sent in MY - ASN in the OPEN message. This will avoid the BGP OPEN Error message - BAD PEER AS, and is typically used to re-establish eBGP sessions with - peers expecting the legacy ASN after a router has been moved to a new - ASN. Additionally, when the BGP speaker configured with this feature - receives updates from its neighbor, it MUST process the update as - normal, but it MUST append the configured ASN in the AS_PATH - attribute before advertising the UPDATE to any other BGP speaker. - Note that processing the update as normal will include appending the - globally configured ASN to the AS_PATH, thus processing this update - will result in the addition of two ASNs to the AS_PATH attribute. - Similarly, for outbound updates sent by the configured BGP speaker to - its neighbor, the speaker MUST append the configured ASN to the - AS_PATH attribute, adding to the existing global ASN in the AS_PATH, - for a total of two ASNs added to the AS_PATH. + When the BGP router receives UPDATEs from its eBGP neighbor + configured with the "Local AS" mechanism, it processes the UPDATE as + described in RFC4271 section 5.1.2 [RFC4271]. However the presence + of a second ASN due to "Local AS" adds the following behavior to + processing UPDATEs received from an eBGP neighbor configured with + this mechanism: - Two options exist to manipulate the behavior of this feature. They - modify the behavior as described below: + 1. Internal: the router MUST append the configured "Local AS" ASN in + the AS_PATH attribute before advertising the UPDATE to an iBGP + neighbor. - No prepend inbound - When the BGP speaker configured with this option - receives inbound updates from its neighbor, it MUST NOT append the - configured ASN in the AS_PATH attribute when advertising that UPDATE - to other peers and instead MUST append only the globally configured - ASN. + 2. External: the BGP router MUST first append the globally + configured ASN to the AS_PATH immediately followed by the "Local + AS" value before advertising the UPDATE to an eBGP neighbor. - No prepend outbound - When the BGP speaker configured with this - option generates outbound BGP updates to the configured peer, the BGP - speaker MUST remove the globally configured ASN from the AS_PATH - attribute, and MUST append the locally configured ASN to the AS_PATH - attribute before sending outbound BGP updates to the configured peer. + Two options exist to manipulate the behavior of the basic "Local AS" + mechanism. They modify the behavior as described below: - While the exact command syntax is an implementation detail beyond the - scope of this document, the following consideration may be helpful - for implementers: Implementations MAY integrate the behavior of the - options described above into a single command that addresses both - inbound and outbound updates, but if this is done, implementations - MUST provide a method to select its applicability to inbound updates, - outbound updates, or updates in both directions. Several existing - implementations use separate commands (e.g. local-as no-prepend vs - local-as replace-as) for maximum flexibility in controlling the - behavior on the session to address the widest range of possible - migration scenarios. + 1. "No Prepend Inbound" - When the BGP router receives inbound BGP + UPDATEs from its eBGP neighbor configured with this option, it + MUST NOT append the "Local AS" ASN value in the AS_PATH attribute + when advertising that UPDATE to iBGP neighbors, but it MUST still + append the globally configured ASN as normal when advertising the + UPDATE to other local eBGP neigbors (i.e. those natively peering + with the globally configured ASN). -4. Internal BGP Autonomous System Migration Features + 2. "Replace Old AS", (outbound) - When the BGP router generates + outbound BGP UPDATEs toward an eBGP neighbor configured with this + option, the BGP speaker MUST NOT (first) append the globally + configured ASN from the AS_PATH attribute. The BGP router MUST + append only the configured "Local AS" ASN value to the AS_PATH + attribute before sending the BGP UPDATEs outbound to the eBGP + neighbor. - The following section describes features that assist with a gradual +4. Internal BGP Autonomous System Migration Mechanisms + + The following section describes mechanisms that assist with a gradual and least service impacting migration of Internal BGP sessions from a - legacy ASN to the permanently retained ASN. The following feature is - very valuable to networks undergoing AS migration, but its use does - not cause changes to the AS_PATH attribute. + legacy ASN to the permanently retained ASN. The following mechanism + is very valuable to networks undergoing AS migration, but its use + does not cause changes to the AS_PATH attribute. 4.1. Internal BGP Alias In this case, all of the routers to be consolidated into a single, permanently retained ASN are under the administrative control of a single entity. Unfortunately, the traditional method of migrating all Internal BGP speakers, particularly within larger networks, is both time consuming and widely service impacting. The traditional method to migrate Internal BGP sessions was strictly limited to reconfiguration of the global configuration ASN and, concurrently, changing all iBGP neighbors' remote ASN from the legacy ASN to the new, permanently retained ASN on each router within the legacy AS. These changes can be challenging to swiftly execute in - networks with with more than a few dozen internal BGP speakers. - There is also the concomitant service interruptions as these changes - are made to routers within the network, resulting in a reset of iBGP + networks with with more than a few dozen internal BGP routers. There + is also the concomitant service interruptions as these changes are + made to routers within the network, resulting in a reset of iBGP sessions and subsequent route reconvergence to reestablish optimal routing paths. Operators often cannot make such sweeping changes given the associated risks of a highly visible service interruption; rather, they require a more gradual method to migrate Internal BGP sessions, from one ASN to a second, permanently retained ASN, that is not visibly service-impacting to its customers. - With the "Internal BGP Alias" [JUNIPER] feature, it allows an - Internal BGP speaker to form a single iBGP session using either the - old, legacy ASN or the new, permanently retained ASN. The benefits - of using this feature are several fold. First, it allows for a more - gradual and less service-impacting migration away from the legacy ASN - to the permanently retained ASN. Second, it (temporarily) permits - the coexistence of the legacy and permanently retained ASN within a - single network, allowing for uniform BGP path selection among all - routers within the consolidated network. NB: Cisco doesn't have an - exact equivalent to "Internal BGP Alias", but the combination of the - Cisco features iBGP local-AS and dual-as provides similar - functionality. + With the "Internal BGP AS Migration" mechanism described herein, it + allows an Internal BGP speaker to form a single iBGP session using + either the old, legacy ASN or the new, permanently retained ASN. The + benefits of using this mechanism are several fold. First, it allows + for a more gradual and less service-impacting migration away from the + legacy ASN to the permanently retained ASN. Second, it (temporarily) + permits the coexistence of the legacy and permanently retained ASN + within a single network, allowing for uniform BGP path selection + among all routers within the consolidated network. - When the "Internal BGP Alias" feature is enabled, typically just on - one side of a iBGP session, it allows that iBGP speaker to establish - a single iBGP session with either the legacy ASN or the new, - permanently retained ASN, depending on which one it receives in the - "My Autonomous System" field of the BGP OPEN message from its iBGP - session neighbor. It is important to recognize that enablement of - the "Internal BGP Alias" feature preserves the semantics of a regular - iBGP session, (using identical ASNs). Thus, the BGP attributes + The iBGP router with the "Internal BGP AS Migration" capability + enabled allows the receipt of a BGP OPEN message with either the + legacy ASN value or the new, globally configured ASN value in the "My + Autonomous System" field of the BGP OPEN message from iBGP neighbors. + It is important to recognize that enablement of the "Internal BGP AS + Migration" mechanism preserves the semantics of a regular iBGP + session, (using identical ASNs). Thus, the BGP attributes transmitted by and the acceptable methods of operation on BGP attributes received from iBGP sessions configured with "Internal BGP - Alias" are no different than those exchanged across an iBGP session - without "Internal BGP Alias" configured, as defined by [RFC4271] and - [RFC4456]. + AS Migration" capability are no different than those exchanged across + an iBGP session without "Internal BGP AS Migration" configured, as + defined by [RFC4271] and [RFC4456]. Typically, in medium to large networks, BGP Route Reflectors [RFC4456] (RRs) are used to aid in reduction of configuration of iBGP sessions and scalability with respect to overall TCP (and, BGP) - session maintenance between adjacent iBGP speakers. Furthermore, BGP + session maintenance between adjacent iBGP routers. Furthermore, BGP Route Reflectors are typically deployed in pairs within a single Route Reflection cluster to ensure high reliability of the BGP Control Plane. As such, the following example will use Route - Reflectors to aid in understanding the use of the "Internal BGP - Alias" feature. Note that Route Reflectors are not a prerequisite to - enable "Internal BGP Alias" and this feature can be enabled - independent of the use of Route Reflectors. + Reflectors to aid in understanding the use of the "Internal BGP AS + Migration" mechanism. Note that Route Reflectors are not a + prerequisite to enable "Internal BGP AS Migration" and this mechanism + can be enabled independent of the use of Route Reflectors. The general order of operations is as follows: 1. Within the legacy network, (the routers comprising the set of devices that still have a globally configured legacy ASN), one member of a redundant pair of RRs has its global configuration - ASN changed to the permanently retained ASN. Concurrently, - "Internal BGP Alias" is configured on all iBGP sessions. This - will comprise Non-Client iBGP sessions to other RRs as well as - Client iBGP sessions, typically to PE devices, both still - utilizing the legacy ASN. Note that during this step there will - be a reset and reconvergence event on all iBGP sessions on the - RRs whose configuration was modified; however, this should not be - service impacting due to the use of redundant RRs in each RR - Cluster. + ASN changed to the permanently retained ASN. Concurrently, the + "Internal BGP AS Migration" capability is enabled on all iBGP + sessions on that device. This will comprise Non-Client iBGP + sessions to other RRs as well as Client iBGP sessions, typically + to PE devices, both still utilizing the legacy ASN. Note that + during this step there will be a reset and reconvergence event on + all iBGP sessions on the RRs whose configuration was modified; + however, this should not be service impacting due to the use of + redundant RRs in each RR Cluster. 2. The above step is repeated for the other side of the redundant pair of RRs. The one alteration to the above procedure is that - "Internal BGP Alias" is now removed from the Non-Client iBGP - sessions toward the other (previously reconfigured) RRs, since it - is no longer needed. "Internal BGP Alias" is still required on - all RRs for all RR Client iBGP sessions. Also during this step, - there will be a reset and reconvergence event on all iBGP - sessions whose configuration was modified, but this should not be - service impacting. At the conclusion of this step, all RRs - should now have their globally configured ASN set to the - permanently retained ASN and "Internal BGP Alias" enabled and in - use toward RR Clients. + the "Internal BGP AS Migration" mechanism is now removed from the + Non-Client iBGP sessions toward the other (previously + reconfigured) RRs, since it is no longer needed. The "Internal + BGP AS Migration" mechanism is still required on all RRs for all + RR Client iBGP sessions. Also during this step, there will be a + reset and reconvergence event on all iBGP sessions whose + configuration was modified, but this should not be service + impacting. At the conclusion of this step, all RRs should now + have their globally configured ASN set to the permanently + retained ASN and "Internal BGP AS Migration" enabled and in use + toward RR Clients. 3. At this point, the network administrators would then be able to establish iBGP sessions between all Route Reflectors in both the legacy and permanently retained networks. This would allow the network to appear to function, both internally and externally, as a single, consolidated network using the permanently retained network. 4. To complete the AS migration, each RR Client (PE) in the legacy network still utilizing the legacy ASN is now modified. Specifically, each legacy PE would have its globally configured - ASN changed to use the permanently retained ASN. The ASN used by - the PE for the iBGP sessions toward each RR would be changed to - use the permanently retained ASN. (It is unnecessary to enable - "Internal BGP Alias" on the migrated iBGP sessions). During the - same maintenance window, External BGP sessions would be modified - to include the above "Local AS No Prepend" and "Replace-AS" - features described in Section 3 above, since all of the changes - are service interrupting to the eBGP sessions of the PE. At this + ASN changed to use the permanently retained ASN. The ASN + configured within the PE for the iBGP sessions toward each RR + would be changed to use the permanently retained ASN. It is + unnecessary to enable "Internal BGP AS Migration" mechanism on + these migrated iBGP sessions. During the same maintenance + window, External BGP sessions would be modified to include the + above "Local AS", "No Prepend" and "Replace Old AS" mechanisms + described in Section 3 above, since all of the changes are + service interrupting to the eBGP sessions of the PE. At this point, all PEs will have been migrated to the permanently retained ASN. - 5. The final step is to excise the "Internal BGP Alias" - configuration from the first half of the legacy RR Client pair -- - this will expunge "Internal BGP Alias" configuration from all - devices in the network. After this is complete, all routers in - the network will be using the new, permanently retained ASN for - all iBGP sessions with no vestiges of the legacy ASN on any iBGP - sessions. + 5. The final step is to excise the "Internal BGP AS Migration" + configuration from the Router Reflectors in an orderly fashion. + After this is complete, all routers in the network will be using + the new, permanently retained ASN for all iBGP sessions with no + vestiges of the legacy ASN on any iBGP sessions. - The benefit of using "Internal BGP Alias" is that it is a more - gradual and less externally service-impacting change to accomplish an - AS migration. Previously, without "Internal BGP Alias", such an AS - migration change would carry a high risk and need to be successfully - accomplished in a very short timeframe (e.g.: at most several hours). - In addition, it would likely cause substantial routing churn and - rapid fluctuations in traffic carried -- potentially causing periods - of congestion and resultant packet loss -- during the period the - configuration changes are underway to complete the AS Migration. On - the other hand, with "Internal BGP Alias", the migration from the - legacy ASN to the permanently retained ASN can occur over a period of - days or weeks with reduced customer disruption. (The only observable - service disruption should be when each PE undergoes the changes - discussed in step 4 above.) + The benefit of using the aforementioned "Internal BGP AS Migration" + capability is that it is a more gradual and less externally service- + impacting change to accomplish an AS migration. Previously, without + "Internal BGP AS Migration", such an AS migration change would carry + a high risk and need to be successfully accomplished in a very short + timeframe (e.g.: at most several hours). In addition, it would + likely cause substantial routing churn and rapid fluctuations in + traffic carried -- potentially causing periods of congestion and + resultant packet loss -- during the period the configuration changes + are underway to complete the AS Migration. On the other hand, with + "Internal BGP AS Migration", the migration from the legacy ASN to the + permanently retained ASN can occur over a period of days or weeks + with reduced customer disruption. (The only observable service + disruption should be when each PE undergoes the changes discussed in + step 4 above.) 4.2. Implementation - When configured with this feature, a BGP speaker MUST accept BGP OPEN - and establish an iBGP session from configured iBGP peers if the ASN - value in MY ASN is either the globally configured ASN or the locally - configured ASN provided in this command. Additionally, a BGP speaker - configured with this feature MUST send its own BGP OPEN using both - the globally configured and the locally configured ASN in MY ASN. To - avoid potential deadlocks when two BGP speakers are attempting to - establish a BGP peering session and are both configured with this - feature, the speaker SHOULD send BGP OPEN using the globally - configured ASN first, and only send a BGP OPEN using the locally - configured ASN as a fallback if the remote neighbor responds with the - BGP error BAD PEER ASN. In each case, the BGP speaker MUST treat - updates sent and received to this peer as if this was a natively - configured iBGP session, as defined by [RFC4271] and [RFC4456]. - - Implementations of this feature MAY integrate the functionality from - the eBGP features (Section 3) section as a part of this command in - order to simplify support for eBGP migrations as well as iBGP - migrations, such that an eBGP session to a configured neighbor could - be established via either the global ASN or the locally configured - ASN. If the eBGP session is established with the global ASN, no - modifications to AS_PATH are required, but if the eBGP session is - established with the locally configured ASN, the modifications - discussed in eBGP features (Section 3) MUST be implemented to - properly manipulate the AS_PATH. + When configured with this mechanism, a BGP speaker MUST accept BGP + OPEN and establish an iBGP session from configured iBGP peers if the + ASN value in "My Autonomous System" is either the globally configured + ASN or a locally configured ASN provided when this capability is + utilized. Additionally, a BGP router configured with this mechanism + MUST send its own BGP OPEN [RFC4271] (see section 4.2) using both the + globally configured and the locally configured ASN in "My Autonomous + System". To avoid potential deadlocks when two BGP speakers are + attempting to establish a BGP peering session and are both configured + with this mechanism, the speaker SHOULD send BGP OPEN using the + globally configured ASN first, and only send a BGP OPEN using the + locally configured ASN as a fallback if the remote neighbor responds + with the BGP error "Bad Peer AS". In each case, the BGP speaker MUST + treat UPDATEs sent and received to this peer as if this was a + natively configured iBGP session, as defined by [RFC4271] and + [RFC4456]. 5. Additional Operational Considerations - This document describes several features to support ISPs and other + This document describes several mechanisms to support ISPs and other organizations that need to perform ASN migrations. Other variations - of these features may exist, for example, in legacy router software + of these mechanisms may exist, for example, in legacy router software that has not been upgraded or reached End of Life, but continues to operate in the network. Such variations are beyond the scope of this document. Companies routinely go through periods of mergers, acquisitions and divestitures, which in the case of the former cause them to accumulate several legacy ASNs over time. ISPs often do not have control over the configuration of customers' devices (i.e.: the ISPs are often not providing a managed CE router service, particularly to medium and large customers that require eBGP). Furthermore, ISPs are using methods to perform ASN migration that do not require coordination with customers. Ultimately, this means there is not a finite period of time after which legacy ASNs will be completely expunged from the ISP's network. In fact, it is common that legacy - ASNs and the associated External BGP AS Migration features discussed - in this document can and do persist for several years, if not longer. - Thus, it is prudent to plan that legacy ASNs and associated External - BGP AS Migration features will persist in a operational network - indefinitely. + ASNs and the associated External BGP AS Migration mechanisms + discussed in this document can and do persist for several years, if + not longer. Thus, it is prudent to plan that legacy ASNs and + associated External BGP AS Migration mechanisms will persist in a + operational network indefinitely. - With respect to the Internal BGP AS Migration Features, all of the + With respect to the Internal BGP AS Migration mechanism, all of the routers to be consolidated into a single, permanently retained ASN are under the administrative control of a single entity. Thus, completing the migration from iBGP sessions using the legacy ASN to the permanently retained ASN is more straightforward and could be accomplished in a matter of days to months. Finally, good operational hygiene would dictate that it is good practice to avoid - using "Internal BGP Alias" over a long period of time for reasons of - not only operational simplicity of the network, but also reduced - reliance on that feature during the ongoing lifecycle management of - software, features and configurations that are maintained on the - network. - -6. Conclusion - - Although the features discussed in this document are not formally - recognized as part of the BGP4 specification, they have been in - existence in commercial implementations for well over a decade. - These features are widely known by the operational community and will - continue to be a critical necessity in the support of network - integration activities going forward. Therefore, these features are - extremely unlikely to be deprecated by vendors. As a result, these - features must be acknowledged by protocol designers, particularly - when there are proposals to modify BGP's behavior with respect to - handling or manipulation of the AS_PATH Attribute. More - specifically, assumptions should not be made with respect to the - preservation or consistency of the AS_PATH Attribute as it is - transmitted along a sequence of ASNs. In addition, proposals to - manipulate the AS_PATH that would gratuitously increase AS_PATH - length or remove the capability to use these features described in - this document will not be accepted by the operational community. + using "Internal BGP AS Migration" capability over a long period of + time for reasons of not only operational simplicity of the network, + but also reduced reliance on that mechanism during the ongoing + lifecycle management of software, features and configurations that + are maintained on the network. -7. Acknowledgements +6. Acknowledgements Thanks to Kotikalapudi Sriram, Stephane Litkowski, Terry Manderson, David Farmer, Jaroslaw Adam Gralak, Gunter Van de Velde, Juan - Alcaide, Jon Mitchell, and Thomas Morin for their comments. + Alcaide, Jon Mitchell, Thomas Morin, Alia Atlas, and Alvaro Retana + for their comments. -8. IANA Considerations +7. IANA Considerations This memo includes no request to IANA. -9. Security Considerations +8. Security Considerations This draft discusses a process by which one ASN is migrated into and subsumed by another. This involves manipulating the AS_PATH Attribute with the intent of not increasing the AS_PATH length, which would typically cause the BGP route to no longer be selected by BGP's - Path Selection Algorithm in others' networks. This could result in a - loss of revenue if the ISP is billing based on measured utilization - of traffic sent to/from entities attached to its network. This could - also result in sudden and unexpected shifts in traffic patterns in - the network, potentially resulting in congestion, in the most extreme - cases. + Path Selection Algorithm in others' networks. This could result in + sudden and unexpected shifts in traffic patterns in the network, + potentially resulting in congestion. - Given that these features can only be enabled through configuration + Given that these mechanisms can only be enabled through configuration of routers within a single network, standard security measures should be taken to restrict access to the management interface(s) of routers - that implement these features. + that implement these mechanisms. Additionally, BGP sessions SHOULD + be protected using TCP Authentication Option [RFC5925] and the + Generalized TTL Security Mechanism [RFC5082] -10. Appendix: Implementation report +9. Appendix: Implementation report - As noted elsewhere in this document, this set of migration features + As noted elsewhere in this document, this set of migration mechanisms has multiple existing implementations in wide use. o Cisco [CISCO] o Juniper [JUNIPER] o Alcatel-Lucent [ALU] This is not intended to be an exhaustive list, as equivalent features do exist in other implementations, however the authors were unable to find publicly available documentation of the vendor-specific implementation to reference. -11. References +10. References -11.1. Normative References +10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. - [RFC5398] Huston, G., "Autonomous System (AS) Number Reservation for - Documentation Use", RFC 5398, December 2008. + [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route + Reflection: An Alternative to Full Mesh Internal BGP + (IBGP)", RFC 4456, April 2006. -11.2. Informative References +10.2. Informative References [ALU] Alcatel-Lucent, "BGP Local AS attribute", 2006-2012, . [CISCO] Cisco Systems, Inc., "BGP Support for Dual AS Configuration for Network AS Migrations", 2003, . [JUNIPER] Juniper Networks, Inc., "Configuring the BGP Local Autonomous System Attribute", 2012, . - [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route - Reflection: An Alternative to Full Mesh Internal BGP - (IBGP)", RFC 4456, April 2006. - [RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous System Confederations for BGP", RFC 5065, August 2007. + [RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C. + Pignataro, "The Generalized TTL Security Mechanism + (GTSM)", RFC 5082, October 2007. + + [RFC5398] Huston, G., "Autonomous System (AS) Number Reservation for + Documentation Use", RFC 5398, December 2008. + + [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP + Authentication Option", RFC 5925, June 2010. + [RFC6996] Mitchell, J., "Autonomous System (AS) Reservation for Private Use", BCP 6, RFC 6996, July 2013. Authors' Addresses Wesley George Time Warner Cable 13820 Sunrise Valley Drive Herndon, VA 20171 US