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AdrianT88
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Joined: Thu Mar 05, 2020 6:20 pm

eBGP routes are not advertised to all neighbors

Fri Feb 25, 2022 5:34 pm

Hi all,

I have a ring network built for redundancy amongst 4 Towers. Tower1 and Tower4 have a direct PtP fiber backhaul to the core network and mmWave PtP to their neighboring towers. I am running eBGP between all the peers and there are no local preferences or weights attached to manually influence the routing. Tower routers are mostly CCR1036 and CCR2004 running OS6.46.8 and 6.48.6.

Logical Topology -
CORE--eBGP--Tower1---eBGP---Tower2---eBGP---Tower3---eBGP---Tower4---eBGP--CORE
|
Tower2 only - New eBGP Peer (ASN65116)

Tower2 has a new eBGP peer and is learning 3 subnets from it - 10.116.50.0/24, 172.25.116.0/30 and 64.85.xx.xx/30. The issue I am having is that Tower2 is advertising subnets 10.116.50.0/24 and 172.25.116.0/30 as expected to Tower1 and Tower3, but subnet 64.85.xx.xx/30 is only advertised to Tower3. I am trying to understand why it is not also advertised to Tower1.

Tower2 learned routes from new peer
[atetu@cedar-rtr01] > ip route print where received-from=eBGP-Urban-League
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
0 ADb 10.116.50.0/24 172.25.116.2 20
1 ADb 64.85.xx.xx/30 172.25.116.2 20
2 ADb 172.25.116.2/32 172.25.116.2 20

Tower2 advertisements to Tower3
[atetu@cedar-rtr01] > ip route print where received-from=eBGP-Urban-League routing bgp advertisements print peer
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
[atetu@cedar-rtr01] > routing bgp advertisements print peer=eBGP-Willson
PEER PREFIX NEXTHOP AS-PATH ORIGIN LOCAL-PREF
eBGP-... 172.25.116.2/32 172.25.111.21 65116 igp
eBGP-... 172.20.105.2/32 172.25.111.21 65105 igp
eBGP-... 10.51.105.0/24 172.25.111.21 65105 igp
eBGP-... 64.85.xx.xx/30 172.25.111.21 65116 igp
eBGP-... 10.116.50.0/24 172.25.111.21 65116 igp
eBGP-... 0.0.0.0/0 172.25.111.21 65105,25814

Tower2 advertisements to Tower1
[atetu@cedar-rtr01] > routing bgp advertisements print peer=eBGP-Riverview
PEER PREFIX NEXTHOP AS-PATH ORIGIN LOCAL-PREF
eBGP-... 10.106.42.0/23 172.25.111.2 65108,65106 igp
eBGP-... 172.20.106.2/32 172.25.111.2 65108,65106 igp
eBGP-... 172.25.116.2/32 172.25.111.2 65116 igp ---------- advertised as expected
eBGP-... 10.116.50.0/24 172.25.111.2 65116 igp ---------- advertised as expected
eBGP-... 10.106.64.0/24 172.25.111.2 65108,65106 igp

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