BGP Graceful Shutdown - IOS XR

2. Advertise routes with a lower preference to allow alternate routes to take over before taking down a link or router. This avoids transient incorrect routing during convergence when a link or router is taken out of service. This is important when BGP convergence takes many seconds or even minutes. BGP Graceful Maintenance overview

3. The Problem DUT Source Destination Alternate 1 2 3 4 1. Take down DUT 2. Packets drop 3. Alternate announces 4. Packets resume A router is unable to forward to a destination if a withdrawal arrives before the advertisement of its alternate route

4. The Solution DUT Source Destination Alternate 1 2 3 4 5. Take down DUT 2. Packets continue 3. Alternate announces 4. Packets reroute 1. Announce Lower Preference 1 5

5. Putting a BGP session into Graceful Maintenance Two things happen 1. Routes from that neighbor are sent to others with GSHUT 2. Routes to that neighbor are sent with GSHUT DUT Gshut Link DUT Gshut Link Route with GSHUT community and possibly other preference lowering attributes added Regular route advertisement

6. GSHUT attribute vs. community Routes received from a GSHUT neighbor are marked with a GSHUT attribute to distinguish them from routes received with the GSHUT community. When a neighbor is taken out of maintenance, the attribute on its paths is removed, but not the community. RP/0/0/CPU0:R1#sh bgp 5.5.5.5/32 ... Local, (Received from a RR-client) 12.12.12.5 from 12.12.12.5 (192.168.0.5) Origin incomplete, metric 0, localpref 100, valid, internal, best, group-best, import-candidate, graceful-shut Received Path ID 0, Local Path ID 1, version 13 ... RP/0/0/CPU0:R4#sh bgp 5.5.5.5 ... 10.10.10.1 from 10.10.10.1 (192.168.0.5) Received Label 24000 Origin incomplete, metric 0, localpref 100, valid, internal, best, group-best, import-candidate Received Path ID 0, Local Path ID 1, version 4 Community: graceful-shutdown Originator: 192.168.0.5, Cluster list: 192.168.0.1 ... Attribute Community

7. Lowering the route preference By default, just add the GSHUT community Optionally, can also add LOCAL_PREF and AS_PATH prepends. Note: This is done after outbound policy is applied. neighbor 666.0.0.1 remote-as 65000 graceful-maintenance local-preference 4 as-prepends 3 Any routes with the GSHUT attribute AND All routes if this neighbor has graceful-maintenance activated will be sent to this neighbor with GSHUT community, LOCAL_PREF=4 and 3 AS_PATH prepends. (LOCAL_PREF will not be sent to EBGP neighbors)

8. Activating graceful-maintenance Globally: for all neighbors neighbor 666.0.0.1 remote-as 65000 graceful-maintenance activate Activate a single neighbor router bgp 65000 graceful-maintenance activate all-neighbors retain-routes Without this, only neighbors with graceful-maintenance configuration will activate RIB will retain BGP routes when BGP process is stopped neighbor-group test graceful-maintenance activate Activate a whole neighbor group

9. graceful-maintenance and Graceful Shutdown The “graceful-maintenance activate” configuration does not disappear after shutdown. It also works at session bringup time. Use the “no” keyword to remove graceful-maintenance activation after bringup is complete. RP/0/0/CPU0:R1(config)#router bgp 1 RP/0/0/CPU0:R1(config-bgp)#no graceful-maintenance RP/0/0/CPU0:R1(config-bgp)# neighbor 666.0.0.1 RP/0/0/CPU0:R1(config-bgp-nbr)#graceful-maintenance RP/0/0/CPU0:R1(config-nbr-gshut)#no activate Removing the global activation For a single neighbor

10. When to Shutdown A router or link can be shut down after the network has converged as a result of a graceful-maintenance activation. Convergence can take from less than a second up to more than an hour. Unfortunately, a single router cannot know when a whole network has converged. After graceful-maintenance activation, it can take a few seconds to start sending updates. Then The “InQ” and “OutQ” of neighbors in the “show bgp summary” command indicates the level of BGP messaging. Neighbors should stop sending traffic to this router. If they do not have alternate routes, they will not stop.

11. Don’t forget Sending GSHUT to another AS can cause churn in far away networks If this router is the only source of a route, then GSHUT does not help. RP/0/0/CPU0:R1(config-bgp)# neighbor 666.0.0.1 RP/0/0/CPU0:R1(config-bgp-nbr)#address-family ipv4 unicast RP/0/0/CPU0:R1(config-bgp-nbr-af)#send-community-gshut-ebgp send-community-gshut-ebgp must be configured under neighbor address family of an EBGP neighbor for this router to add the GSHUT community. This does not affect a GSHUT community that was received on a route from another source. For that, use send-community-ebgp.

12. References 1. http://tools.ietf.org/html/draft-ietf-grow-bgp-gshut-06 2. http://tools.ietf.org/html/rfc6198

13. Thank you.

Bertrand Duvivier

BGP Graceful Shutdown - IOS XR