When inspecting captured netlink messages on wireshark, I noticed that first a “route add” message is sent with flags CREATE and EXCL with my other router as the gateway. This is the route from the iBGP session which ends up in the VPP FIB. Then a second “route add” message is sent with flags CREATE and REPLACE, with the uplink peer router as the gateway. This route is not installed in the VPP FIB, though it is in the kernel routing table, as I showed in the previous email.
I’m not sure if LCP is simply not getting the second message or if it gets it but doesn’t install the route due to some error. Thanks, Andre > Em 11 de ago. de 2025, à(s) 16:24, Andre Nathan via lists.fd.io > <andre=digirati.com...@lists.fd.io> escreveu: > > Hi > >> On 8/8/25 3:14 PM, Matthew Smith via lists.fd.io wrote: >> You could try adding del-dynamic-on-link-down to the linux-cp section of >> your startup.conf and see if that helps. > > I've tried this parameter, but it didn't make a difference when bringing the > iBGP session down. > > Now that I'm inspecting this more closely, I've found some strange behavior > where the VPP routes don't match the ones installed by Bird. > > In this case, I've started bird with the uplink BGP session enabled, and > afterwards started the iBGP session. I've noticed the VPP FIB has ~24k routes > pointing to my other router, that is, learned via the iBGP session. This is > weird because routes via the other router should only be used in case the BGP > session with the uplink peer goes down, as they by definition have one extra > hop, and are therefore "worse". > > I've checked that bird is indeed selecting the uplink route as the preferred > one. One example: > > # birdc show route for 45.181.63.0/24 > BIRD 2.16.1 ready. > Table master4: > 45.181.63.0/24 unicast [uplink_ipv4 17:44:31.966] * (100) [AS269170i] > via x.y.z.w on uplink1 > unicast [ibgp_ipv4 17:44:31.609] (100) [AS269170i] > via a.b.c.d on external > > The kernel route is in agreement: > > # ip route show 45.181.63.0/24 > 45.181.63.0/24 via x.y.z.w dev uplink1 proto bird metric 32 > > But the route in VPP is through my other router (a.b.c.d): > > # vppctl show ip fib 45.181.63.0/24 > ipv4-VRF:0, fib_index:0, flow hash:[src dst sport dport proto flowlabel ] > epoch:0 flags:none locks:[adjacency:1, default-route:1, lcp-rt:1, ] > 45.181.63.0/24 fib:0 index:18181 locks:2 > lcp-rt-dynamic refs:1 src-flags:added,contributing,active, > path-list:[77] locks:49272 flags:shared,popular, uPRF-list:69 len:1 > itfs:[7, ] > path:[103] pl-index:77 ip4 weight=1 pref=32 attached-nexthop: > oper-flags:resolved, > a.b.c.d BondEthernet0 > [@0]: ipv4 via a.b.c.d BondEthernet0: mtu:1500 next:8 flags:[] > 12b9c288223f3cfdfe9ec4e40800 > > forwarding: unicast-ip4-chain > [@0]: dpo-load-balance: [proto:ip4 index:18182 buckets:1 uRPF:69 to:[0:0]] > [0] [@5]: ipv4 via a.b.c.d BondEthernet0: mtu:1500 next:8 flags:[] > 12b9c288223f3cfdfe9ec4e40800 > ipv4-VRF:180, fib_index:1, flow hash:[src dst sport dport proto flowlabel ] > epoch:0 flags:none locks:[lcp-rt:1, ] > 0.0.0.0/0 fib:1 index:49 locks:2 > default-route refs:1 entry-flags:drop, src-flags:added,contributing,active, > path-list:[66] locks:2 flags:drop, uPRF-list:62 len:0 itfs:[] > path:[92] pl-index:66 ip4 weight=1 pref=0 special: cfg-flags:drop, > [@0]: dpo-drop ip4 > > forwarding: unicast-ip4-chain > [@0]: dpo-load-balance: [proto:ip4 index:50 buckets:1 uRPF:62 to:[0:0]] > [0] [@0]: dpo-drop ip4 > > > > I don't really know how to explain that. > > > > > >
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