はじめに
本ドキュメントでは、IOS-XR における BGP Graceful Restart (GR) における Helper Mode の動作について説明します。
まず、簡単に GR について説明します。GR は、BGP セッションを再開中に、BGP ルーティングにおけるピアがトラフィックの転送を継続する仕組みです。具体的には、BGP の再開中にピアから学習した経路の状態が保持され、stale としてマークされます。
BGP Graceful Restart における Helper Mode
7.1.2, 7.2.1, 6.7.2 より前のバージョンでは、OPEN メッセージにてピアが GR Capability と呼ばれる BGP Capability を送ってきている場合、ローカルルータにて GR を設定していなくても Helper Mode として動作し、ピアから学習した経路を保持します。
バージョン 7.1.2, 7.2.1, 6.7.2 以降では CSCvo71226 による機能拡張にて、BGP ネイバーセッションがダウンした際に、ローカルの BGP スピーカーにて GR が設定されていない状況下で、bgp graceful-restart retain-nbr-routes disable が設定されている場合、そのローカルスピーカーは、ネイバーから受信した経路を保持しません。これは、Cisco ドキュメントにて GR Helper Mode Preemption と呼ばれています。
設定例
GR Helper Mode Preemption の設定例を以下に記載します。
RP/0/RSP0/CPU0:ios(config)# router bgp 100
RP/0/RSP0/CPU0:ios(config-bgp)# bgp graceful-restart
RP/0/RSP0/CPU0:ios(config-bgp)# bgp graceful-restart retain-nbr-routes disable
参考情報
RFC 4724
https://datatracker.ietf.org/doc/html/rfc4724
4. Operation
A BGP speaker MAY advertise the Graceful Restart Capability for an address family to its peer if it has the ability to preserve its forwarding state for the address family when BGP restarts. In addition, even if the speaker does not have the ability to preserve its forwarding state for any address family during BGP restart, it is still recommended that the speaker advertise the Graceful Restart Capability to its peer (as mentioned before this is done by not including any <AFI, SAFI> in the advertised capability).
4.1. Procedures for the Restarting Speaker
When the Restarting Speaker restarts, it MUST retain, if possible, the forwarding state for the BGP routes in the Loc-RIB and MUST mark them as stale. It MUST NOT differentiate between stale and other information during forwarding.
CSCvo71226 - XR not sending BGP graceful restart capability in open message for helper mode only
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvo71226?rfs=iqvred
Routing Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 7.1.x
https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r7-1/routing/configuration/guide/b-routing-cg-asr9000-71x/b-routing-cg-asr9000-71x_chapter_010.html#Cisco_Concept.dita_45ee4554-6fd1-47a7-8560-1eb4b55adde2
Graceful Restart
The BGP graceful restart helper mode feature is available on a global basis. Graceful restart is the mechanism by which BGP routing peers continue to forward traffic while the BGP session is restarting. If a BGP session is already established with a neighbor and the neighbor requests a new session, the new session is accepted if graceful restart is configured for that neighbor on the local BGP speaker. There must also be one AFI/SAFI in the Graceful Restart Capability in the OPEN message received from that neighbor's existing session; otherwise, the old session is retained.
Graceful Restart Helper Mode Preemption
In the current implementation, when a BGP session goes down, the routes are preserved and marked as stale for every address family for which an AFI/SAFI exists in the Graceful Restart Capability received in the BGP OPEN Message from that neighbor. Now, when a BGP neighbor session goes down, and it is not configured for graceful-restart and bgp graceful-restart retain-nbr-routes disable is configured, then the local BGP speaker does not retain the routes received from that neighbor.
