bgp problem:

[att-sgcops]

4 routers as u can see from the map, only BGP routing protocol trun on. I advertise 22.22.22.0/24 from R2:

_____________________________________

R1#sh run | b router bgp

router bgp 100

no synchronization

bgp log-neighbor-changes

neighbor 12.12.12.2 remote-as 200

neighbor 41.41.41.4 remote-as 100

no auto-summary

____________________________________

R2#sh run | b router bgp

router bgp 200

bgp log-neighbor-changes

network 22.22.22.0 mask 255.255.255.0

neighbor 12.12.12.1 remote-as 100

neighbor 23.23.23.3 remote-as 300

____________________________________

R3#sh run | b router bgp

router bgp 300

no synchronization

bgp log-neighbor-changes

neighbor 23.23.23.2 remote-as 200

neighbor 43.43.43.4 remote-as 100

no auto-summary

_____________________________________

R4#sh run | b router bgp

router bgp 100

no synchronization

bgp log-neighbor-changes

neighbor 41.41.41.1 remote-as 100

neighbor 43.43.43.3 remote-as 300

no auto-summary

_____________________________________

I found R2 advertised this network to R1 and R3, and R1 and R4 advertised it each other, but R3 advertise it to R4, why R4 not advertised it back to R3 as it did to R1?

____________________________________________________________

R1#sh ip bgp 22.22.22.0

BGP routing table entry for 22.22.22.0/24, version 2

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Advertised to update-groups:

2

300 200

43.43.43.3 (inaccessible) from 41.41.41.4 (33.33.33.36)

Origin IGP, metric 0, localpref 100, valid, internal

200

12.12.12.2 from 12.12.12.2 (22.22.22.22)

Origin IGP, metric 0, localpref 100, valid, external, best

______________________________________________________________

R4#sh ip bgp 22.22.22.0

BGP routing table entry for 22.22.22.0/24, version 2

Paths: (2 available, best #1, table Default-IP-Routing-Table)

Advertised to update-groups:

1

300 200

43.43.43.3 from 43.43.43.3 (140.231.116.49)

Origin IGP, localpref 100, valid, external, best

200

12.12.12.2 (inaccessible) from 41.41.41.1 (20.20.20.1)

Origin IGP, metric 0, localpref 100, valid, internal

______________________________________________________________

R3#sh ip bgp 22.22.22.0

BGP routing table entry for 22.22.22.0/24, version 2

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Advertised to update-groups:

1

200

23.23.23.2 from 23.23.23.2 (22.22.22.22)

Origin IGP, metric 0, localpref 100, valid, external, best

_______________________________________________________________

[att-sgcops]

and after I apply RIP routing on every router:

R3#sh run | b router rip

router rip

version 2

network 21.0.0.0

network 43.0.0.0

no auto-summary

R4#sh run | b router rip

router rip

version 2

network 41.0.0.0

network 43.0.0.0

no auto-summary

R1#sh run | b router rip

router rip

version 2

network 12.0.0.0

network 41.0.0.0

no auto-summary

R2#sh run | b router rip

router rip

version 2

network 12.0.0.0

network 23.0.0.0

no auto-summary

I found bgp routing table changed: now R4 start to advertise 22.22.22.0/24 with R3 each other and it stop advertise it to R1:

_____________________

R1#sh ip bgp 22.22.22.0

BGP routing table entry for 22.22.22.0/24, version 2

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Advertised to update-groups:

2

200

12.12.12.2 from 12.12.12.2 (22.22.22.22)

Origin IGP, metric 0, localpref 100, valid, external, best

___________________________

R4#sh ip bgp 22.22.22.0

BGP routing table entry for 22.22.22.0/24, version 3

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Advertised to update-groups:

2

300 200

43.43.43.3 from 43.43.43.3 (140.231.116.49)

Origin IGP, localpref 100, valid, external

200

12.12.12.2 (metric 1) from 41.41.41.1 (20.20.20.1)

Origin IGP, metric 0, localpref 100, valid, internal, best

__________________________

R3#sh ip bgp 22.22.22.0

BGP routing table entry for 22.22.22.0/24, version 2

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Advertised to update-groups:

1

100 200

43.43.43.4 from 43.43.43.4 (33.33.33.36)

Origin IGP, localpref 100, valid, external

200

23.23.23.2 from 23.23.23.2 (22.22.22.22)

Origin IGP, metric 0, localpref 100, valid, external, best

________________________________

can someone tell me the reason?

[kamlesh.sharma]

from R1 and R4 you need to put command neighbor next-hop-self command so that links conneceted to R4-R3 and R1-R2 is reachable via IGP in as 100 R1 and R4.

as in ur output on R1 it says 43.43.43.0 is inaccessible and on R4 12.12.12.0 is inaccessible.

HTH

[att-sgcops]

i know there is no next-hop-self cause the next hop inacessible, my question is: why r4 advertised with r1 each other when there is not IGP, and r4 advertised the network with r3 each other when there is IGP (RIP in this case)?

[chuongnh]

I know your mean, R1 and R4 is connected in the same AS 100. You can see it when use command: show ip route

[att-sgcops]

thanks for your quick reply, but i think my problem is why. I can see the bgp table using "show ip bgp". Please explain to me why r4 stop advertise to r1 and begin to advertise with r3 each other after implement rip.

[viettel_adm]

You wonder about not use RIP but routing information still exchange between R1 and R4. The reason is IGP here use connected status. For more detail, please use command: show ip route

[chuongnh]

Dear Mate,

The reason is you forgot config next-hop-self:

R4(config-router)#neighbor 41.41.41.1 next-hop-self

R1(config-router)#neighbor 41.41.41.4 next-hop-self

Try again with new config ...

Please vote for me !

[olorunloba · Accepted solution]

What you need to note is that BGP only advertises routes that are in the routing table. A BGP learnt prefix that is not the best route (hence not in the routing table) will not be advertised. The same also applies with BGP learnt prefixes that are inaccessible.

The note also that routes learnt from exrernal peers are advertised to all peers, internal and external. Routes learnt from internal peers are advertised only to external peers.

In the first situation. R4 recieves the route from 2 sources, one from R1 and the other from R3. The one from R1 has its next-hop inaccessible, it therefore cannot be entered into the routing table. The best route is therefore the one from R3. R4 therefore advertises the route to all her peers, i.e. R1 and R3. R1 enters it into its BGP table, but R3 does not because of AS-Path loop.

In the second scenario, R4 still recieves two route, but the one from R1 is the best, as the next-hop is now accessible (thanks to you enabling RIP). Becasue R1 is an internal peer, it is not advertised to internal peers (R1 inclusive).

hope this helps