BGP route advertisement

[MOHAMMED SALMAN]

Hi All,

I have got a topology as shown below:

R16---IBGP----R18

|                          |

EBGP                 EBGP

|                          |  

R23--IBGP-----R20

Here R20 is learning a route 192.168.0.0/16 which is generated by R16. The route should be learned from R23 but the show output has totally confused me. Please explain me

R20#sho ip bgp sum
BGP router identifier 10.0.20.20, local AS number 200
BGP table version is 3, main routing table version 3
1 network entries using 120 bytes of memory
1 path entries using 52 bytes of memory
2/1 BGP path/bestpath attribute entries using 248 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
Bitfield cache entries: current 1 (at peak 2) using 32 bytes of memory
BGP using 476 total bytes of memory
BGP activity 1/0 prefixes, 2/1 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.23.23 4 200 42 42 3 0 0 00:37:56 1
10.0.24.24 4 200 41 42 3 0 0 00:37:57 0
10.0.80.18 4 100 45 45 3 0 0 00:13:44 0

R20#sho ip bgp neigh 10.0.80.18 advertised-routes
BGP table version is 3, local router ID is 10.0.20.20
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
*>i192.168.0.0/16 10.0.63.16 0 100 0 100 ?

Total number of prefixes 1

R20#sho ip bgp neigh 10.0.23.23 advertised-routes

Total number of prefixes 0

R20#sho ip bgp 192.168.0.0
BGP routing table entry for 192.168.0.0/16, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to update-groups:
1
100
10.0.63.16 (metric 2172416) from 10.0.23.23 (10.0.23.23)
Origin incomplete, metric 0, localpref 100, valid, internal, best
R20#

Here sho ip bgp neigh 10.0.80.18 advertised-routes  shows that the route 192.168.0.0/16 has been advertised by R18 and  sho ip bgp 192.168.0.0  tells that it is learned from 10.0.23.23

[paul driver]

Hello

This  down to bgp path selection,,,, if all things are equal bgp will select ebgp over ibgp advertised prefix.

res

Paul

[Masoud Pourshabanian]

Hello,

R20 should receive two entries for 192.168.0.0.  One from R18 and one from R23. Since route received from EBGP is preferred over IBGP, R20 should install the route from R18 into routing table.

Please share the output of these commands to make sure R16 and R18 are advertising that route

on R16-->>Show IP bgp neigh R18 advertised-routes

on R18-->>Show IP bgp neigh R20 advertised-routes

[paul driver]

Hello Masoud

R20 should install the route from R18 into routing table.

Humm...Only in the bgp adj-rib-in table and not in the local rib table as the bgp best path selection will just enter the best route ?

So you'll see ii in sh ip bgp output but not in the sh ip route output

res
Paul

[Masoud Pourshabanian]

Hello,

There will be two entries for 192.168.0.0 in the BGP table but only one entry will be installed in the routing table since the BGP path selection selects route received from R18 as the best route.

But in the output of "R20#sho ip bgp 192.168.0.0"  I only see one entry so I guess there is a problem with advertising that route from R16 to R18 and then to R20.

best,

Masoud

[paul driver]

Hello

The reason for that it looks like the OP topology isn  true to the OP bgp summary post.
An additional IBGP peering can be seen 10.0.24.24

R20#sho ip bgp sum

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.23.23 4 200 42 42 3 0 0 00:37:56 1
10.0.24.24 4 200 41 42 3 0 0 00:37:57 0
10.0.80.18 4 100 45 45 3 0 0 00:13:44 0

[Masoud Pourshabanian]

I supposed 10.0.24.24  was hanging off of R20 downward. If It is not the case it might be the other reason.

[MOHAMMED SALMAN]

You got it right here.. R24 is downward but as the problem was not with it, I did not include it in the topology

[paul driver]

Hello

10.0.63.16 (metric 2172416) from 10.0.23.23 (10.0.23.23)
Origin incomplete, metric 0, localpref 100, valid, internal, best

 Are you performing any redistribution for this route?

Can you post the bgp route routes and advertised routes output from R16 neighbour 

Sh ip bgp neighbor xxxx routes

sh ip bgp neighbor xxxx advertised-routes

sh ip route 192.168.0.0

res

paul

[MOHAMMED SALMAN]

Masoud,

I agree with your point but I have enabled synchonization and thus R18 should not advertise the route to R20

[Jon Marshall]

I agree with Mohammed here, the reason seems to be with R18 not advertising the route.

It will not be because of the route advertisement it receives from R20 because it should reject that route as it will see it's own AS unless of course you have added extra configuration to allow that.

A "sh ip bgp 192.168.0.0" may well show the next hop as inaccessible which would mean it could not advertise it to R20 so R20 can then advertise the route to R18.

Jon

[MOHAMMED SALMAN]

Jon,

As per my config an understanding R16 advertises the route to R18 and R23. However R18 would not forward this route to R20 as synchorization is ON and R18 wold not forward a route learned by IBGP to an EBGP.

The confusion arises when I run the sho ip bgp neigh 10.0.80.18 advertised-routes  and sho ip bgp 192.168.0.0  on R20 as shown in the outputs earlier shown in the post.

[Jon Marshall · Accepted solution]

Why the confusion ?

If R18 is not advertising to R20 then R20 can advertise to R18

Jon

[MOHAMMED SALMAN]

So stupid of me.

R20#sho ip bgp neigh 10.0.23.23 advertised-routes

This commands shows the networks that are advertised outside and not the one that are received!!! 

Am I right???

[paul driver · Accepted solution]

Hello

Yes that is correct.