Hi Jakob. The simple configuration below allows communities to be forwarded (send-community-ebgp), but are cleaned at egress (using route-policy and community-set).
In the experiment, the router receives announcements with altering community attributes only, from the internal peer. After the filter is applied, the router sends duplicates to the external peer. Also, In a slightly different setup, the router sends duplicates due to changes in the next-hop only. best regards Thomas --- RP/0/0/CPU0:ios(config)#show running-config Tue Oct 20 02:56:24.230 UTC Building configuration... !! IOS XR Configuration 6.0.1 !! Last configuration change at Tue Oct 20 02:56:02 2020 by cisco ! interface MgmtEth0/0/CPU0/0 shutdown ! interface GigabitEthernet0/0/0/0 ipv4 address 10.12.0.2 255.255.255.252 ! interface GigabitEthernet0/0/0/1 ipv4 address 10.20.0.1 255.255.255.252 ! community-set all *:* end-set ! route-policy nofilter pass end-policy ! route-policy egressfilter delete community in all pass end-policy ! router bgp 65002 bgp router-id 10.12.0.2 address-family ipv4 unicast ! neighbor 10.12.0.1 remote-as 65001 address-family ipv4 unicast send-community-ebgp route-policy egressfilter out ! neighbor 10.20.0.2 remote-as 65002 address-family ipv4 unicast ! end On 10/17/20 3:59 PM, Jakob Heitz (jheitz) via NANOG wrote: > IOS-XR has duplicate update suppression logic for EBGP sessions, > not for IBGP sessions. > > If you are using EBGP and seeing a fault in the duplicate update > suppression logic in IOS-XR, please let me know configs and details > of the experiment. > > Regards, > Jakob. > > -----Original Message----- > Date: Thu, 15 Oct 2020 18:35:58 -0700 > From: Thomas Krenc <tkr...@nps.edu> > > Dear NANOG, > > As a team of researchers from NPS and TU Berlin, we are investigating > the impact of BGP community attributes on the update behavior between ASes. > > We find that when a route is associated with multiple distinct community > attributes it does not only lead to multiple announcement at the tagging > AS, but also at neighboring ASes, if communities are not filtered > properly. This behavior is wide-spread. > > In order to better understand our observations, we have performed a > series of laboratory experiments using Cisco IOS, Junos OS, as well as > the BIRD daemon. > > We find that - by default - all tested routers generate announcements > with changing community attributes, even when other attributes do not > change. In addition, when communities are filtered at egress, Cisco und > BIRD send duplicate announcements (Juniper does not). > > Since our findings are limited to observations in public data as well as > few router implementations, we would like to share our research and > kindly ask you to have a look at: > > ??? https://www.cmand.org/communityexploration/ > > There, we provide some resources documenting our research, as well as > open questions. We greatly appreciate any feedback and insights you can > offer. Also, please don't hesitate to contact us directly: > > ??? communityexploration AT cmand DOT org > > best regards > > Thomas Krenc > Postdoctoral Researcher > Naval Postgraduate School