Description

When Route Target Filtering/Constraint is configured (Juniper reference document: https://www.juniper.net/documentation/us/en/software/junos/vpn-l3/topics/topic-map/l3-vpns-route-target-filtering.html), PE routers advertise route-target prefixes to BGP Route Reflectors (RRs) to reduce the number of received VPNv4 and VPNv6 routes.

 

If multiple PE routers advertise the same prefixes, the router with the lowest Router-ID is selected as the best path. In a corner case where this best PE router loses its BGP connection to the RR, the remote cluster RR may detect the failure first due to race conditions and hide the corresponding route-target prefixes. At the same time, it triggers UNREACH updates for all related VPNv4 and VPNv6 prefixes to the local cluster RR. These UNREACH updates then propagate to all local PE routers, causing the withdrawal of related inter-cluster VPNv4/VPNv6 routes from the remote cluster RR.

 

Subsequently, the local cluster RR detects the PE router failure and reselects the best path for the route-target prefixes, leading to the reinstallation of all related VPNv4 and VPNv6 routes. However, a brief outage may occur during this transition.

 

 

Symptoms

The following command output shows that router a.b.c.d was selected as the best path for prefix XXX:YYY:

  • show route table bgp.rtarget.0 match-prefix "*XXX:YYY*" extensive

When the router a.b.c.d lost its BGP connection to the local cluster RR, triggering a refresh of all related VPNv4/VPNv6 routes (withdrawal and reinstallation), please execute the following command:

  • show route table bgp.l3vpn.0 match-prefix "*XXX:YYY*"

You will see that the VPNv4 route age has been reset to 00:00:00.

 

Solution

It is recommended:

" In all places where Route Target Constraint (RTC) is configured, the additional knob nexthop-resolution no-resolution will be included for the purposes of bypassing nexthop resolution for RIB installation. This configuration is useful for routes not used for forwarding, e.g., non-forwarding RRs. "

Sample configuration:

set protocols bgp group <RR_PEERS> family route-target nexthop-resolution no-resolution

Modification History

2025-03-03 : Article Created