BGP with prefix-list based route-maps - more specific routes?

[sergiu.panaite]

Hello! I hope this isn't a thoroughly newbie-like question, but I wasn't able to find an obvious, clear answer in the usual documentation and resources.

The scenario is as follows: two routers peering via eBGP; router A has a more generic /28 route via one host, and a specific /32 (which is part of the /28) via another. The prefix-list out for A to B only allows the /28.

Does this mean that traffic destined for the /32 host from (or from behind) router B will not be routed by router A via the more specific /32 route? If so, why, since router B does have a less specific but nevertheless valid route for the entire /28?

Config example - might not be 100% accurate, but it should reflect the real-life scenario. I can confirm that the BGP peering is up, that router B sees the 10.10.10.0/28 route and accepts it into its routing table.

ISSUE: router B (and hosts behind it) cannot reach 10.10.10.9.

MY SUSPICION: that "ip prefix-list routerB_bgp_out" would need to also have "10.10.10.9/32" or "ge 32" for example.

Router A:

router bgp 65143
 bgp router-id 192.168.1.1
 bgp log-neighbor-changes
 neighbor 192.168.100.2 remote-as 65400
 neighbor 192.168.100.2 description RouterB_BGP
 neighbor 192.168.100.2 update-source Loopback0
 !
 address-family ipv4
  neighbor 192.168.100.2 activate
  neighbor 192.168.100.2 soft-reconfiguration inbound
  neighbor 192.168.100.2 route-map routerB_bgp_in in
  neighbor 192.168.100.2 route-map routerB_bgp_out out
  no auto-summary
  no synchronization
  network 10.10.10.0 mask 255.255.255.240
  
  
route-map routerB_bgp_in permit 10
 match ip address prefix-list routerB_bgp_in

route-map routerB_bgp_out permit 10
 match ip address prefix-list routerB_bgp_out
 
ip prefix-list routerB_bgp_out permit 10.10.10.0/28

ip prefix-list routerB_bgp_in permit 10.50.50.0/27

ip route 10.10.10.0 255.255.255.240 192.168.200.1
ip route 10.10.10.9 255.255.255.255 192.168.200.20

SHOW:
RouterA#sh ip route 10.10.10.0
Routing entry for 10.10.10.0/28, 2 known subnets
  Variably subnetted with 2 masks

S       10.10.10.9/32 [1/0] via 192.168.200.20
S       10.10.10.0/28 [1/0] via 192.168.200.1

RouterA#sh ip bgp neighbors 192.168.100.2 advertised-routes
BGP table version is 6109, local router ID is 192.168.100.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 10.10.10.0/28  192.168.200.1               0         32768 i

I hope this makes sense.

Thank you for any help!

[Krishnendu AR]

Hi Sergiu,

Can you please share the routing table output from RouterB to see if it receives the BGP routes and add it into the routing table.

If it has a route 10.10.10.0/28 route in its routing table, it will try to reach 10.10.10.9 via this route unless it has a better route from some other source.

If the routing at RouterB is fine, then please check if 10.10.10.9 has the return route to reach the RouterB IP address. Check the default gateway set in host 10.10.10.9.

Krishna

[Richard Burts]

I agree with Krishna that with this config the route to 10.10.10.0/28 should be advertised to router B and that if router B has installed that route into its routing table then it should try to get to 10.10.1.9 via router A.

So the first step is to verify whether router B has installed that route into its routing table. The second step is to verify whether 10.10.10.9 has a route back to the source address at router B (and to verify that router A has a route to the source address also).

HTH

Rick

[paul driver · Accepted solution]

Hello

rtr A has a valid prefix in its bgp and route tables so would be happy to advertise your /20

Your prefix-list also looks okay and I agree with the other guys - Probably 192.168.200.20 hasn't a route back towards rtr A

FYI - for a summary advertisement just the network command of the host prefix and a aggregate would also work

router bgp xxx
network 10.10.10.9 mask 255.255.255.255
aggregate-address 10.10.10.0 255.255.255.240 summary-only as-set

res

Paul