A simple bgp question

[ilovebgp4]

Hi!Guys,I am a newbie in BGP area,I have a doubt about IGP/IBGP Synchronization and want your help.

Does it means if the below conditions satisfied the bgp router will always advertise the bgp route to ebgp peers?even if there is no "network" or "redistribue" command to declare it.

conditions:

1.the router's bgp table has the bgp route.

2.the router's igp route table has the route,that means the router learns the bgp route through igp(border bgp router redistribute all bgp route into igp).

Thanks for your help!

[pkhatri]

That is correct.

The reason you don't need a redistribute or network statement in this case is because you are simply relaying the route through. The redistribute or network statements are used to originate routes into BGP, and are not required if you are re-advertising a route that you learned via BGP in the first place.

Hope that helps - pls rate the post if it does.

Paresh

[ilovebgp4]

your comment is very clear.Thanks a lot!

BTW,I want confirm that in this situation the "synchronization" should be enable safely(by " syn" command),right?cause IBGP is running over IGP.

But when I done the lab for TCP/IP Volumn II(by jeff doyle) chapter 3 "configing and troubleshooting BGP4" figure 3-6 "IBGP over an IGP",the EBGP Peer(Alta) can not get the bgp routing information from it(Telluride),It's strange,I'll check it again.

[balajitvk]

Hi,

When an AS provides transit service to other ASs and if there are non-BGP routers in the AS, transit traffic might be dropped if the intermediate non-BGP routers have not learned routes for that traffic via an IGP.

The BGP synchronization rule states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP.

Synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later. So in that u have manually enable it by using Synchronization Command under router configuration.

In your case pls be ensure whether ur ebgp router is directly connected or not.Otherwise u should use neighbor ebgp-multihop command.

Hope it will help,

Rgs

[ilovebgp4]

In this case the ebgp router is directly connected.

thanks a lot for your help,

[balajitvk]

Hi,

If you can send a topology & config or a link as reference, that will be helpful to deal further.

Rgs,

[ilovebgp4]

pls see the attachment topology;below is the configration:

Vail:

...

router ospf 100

redistribute bgp 100 subnets

network 192.168.1.221 0.0.0.0 area 0

!

router bgp 100

synchronization

bgp log-neighbor-changes

neighbor 192.168.1.197 remote-as 100

neighbor 192.168.1.197 next-hop-self

neighbor 192.168.1.210 remote-as 300

neighbor 192.168.1.225 remote-as 200

no auto-summary

!

Aspne:

...

router ospf 100

log-adjacency-changes

network 192.168.1.0 0.0.0.255 area 0

Telluride:

...

router ospf 100

redistribute bgp 100 subnets

network 192.168.1.197 0.0.0.0 area 0

!

router bgp 100

synchronization

bgp log-neighbor-changes

neighbor 192.168.1.205 remote-as 400

neighbor 192.168.1.221 remote-as 100

neighbor 192.168.1.221 next-hop-self

no auto-summary

[ilovebgp4]

below is the information from Telluride

Telluride#sh ip bgp

BGP table version is 4, local router ID is 192.168.1.206

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

*> 192.168.1.200/30 192.168.1.205 0 0 400 i

* i192.168.1.212/30 192.168.1.221 0 100 0 300 i

* i192.168.1.216/30 192.168.1.221 0 100 0 200 i

* i192.168.1.224/30 192.168.1.221 0 100 0 200 i

*> 192.168.50.0 192.168.1.205 0 0 400 i

*> 192.168.75.0 192.168.1.205 0 0 400 i

* i192.168.100.0 192.168.1.221 0 100 0 200 i

* i192.168.200.0 192.168.1.221 2195456 100 0 200 i

* i192.168.250.0 192.168.1.221 0 100 0 300 i

notice:here some bgp entry did not has a ">" beside it,means it's not the best route,why?

Telluride#sh ip route

Gateway of last resort is not set

B 192.168.75.0/24 [20/0] via 192.168.1.205, 02:29:54

O E2 192.168.200.0/24 [110/1] via 192.168.1.198, 00:45:49, Serial0

O E2 192.168.250.0/24 [110/1] via 192.168.1.198, 00:45:49, Serial0

B 192.168.50.0/24 [20/0] via 192.168.1.205, 02:29:54

192.168.1.0/30 is subnetted, 6 subnets

B 192.168.1.200 [20/0] via 192.168.1.205, 02:29:54

C 192.168.1.204 is directly connected, Serial1

C 192.168.1.196 is directly connected, Serial0

O E2 192.168.1.216 [110/1] via 192.168.1.198, 00:45:50, Serial0

O 192.168.1.220 [110/128] via 192.168.1.198, 00:45:50, Serial0

O E2 192.168.1.212 [110/1] via 192.168.1.198, 00:33:41, Serial0

O E2 192.168.100.0/24 [110/1] via 192.168.1.198, 00:45:50, Serial0

And below is the error from Telluride's console momently:

02:32:59: BGP(0): no valid path for 192.168.1.212/30

02:32:59: BGP(0): no valid path for 192.168.1.216/30

02:32:59: BGP(0): no valid path for 192.168.1.224/30

02:32:59: BGP(0): no valid path for 192.168.100.0/24

02:32:59: BGP(0): no valid path for 192.168.200.0/24

02:32:59: BGP(0): no valid path for 192.168.250.0/24

[nethelper]

Hello,

your configs look ok. Have you reset all the BGP sessions and OSPF processes ? Better yet, I would reboot all three routers, just make sure to save the running to startup config first.

Regards,

Nethelper

[pkhatri]

Could you post the output of 'sh ip bgp 192.168.1.212 255.255.255.252' from Telluride ?

Paresh

[balajitvk]

Hi,

From your show ip bgp output

* i192.168.1.212/30 192.168.1.221 0 100 0 300 i

* i192.168.1.216/30 192.168.1.221 0 100 0 200 i

* i192.168.1.224/30 192.168.1.221 0 100 0 200 i

it is clear that those routes are there in bgp table but not considered as best to put it in the routing table.

just try this command show ip bgp prefix command for all these three routes and see whether the output shows synchronized or not synchronized.

if it says not synchronized, then

try by disabling synchronization and also check the reachablity of nexthop that is given in that output.

Because if the nexhop is not reachable it will put the route in BGP table but not in routing table.

Hope this will clears ur probs,

Rgs,

[ilovebgp4]

Yeah,you are right.I have noticed these 3 bgp entries haven't a ">" beside it and it's not the best route.But what's the reason?Cause they are the only routes to the destinationes,if they are not the best,which one is the best?

And disabling synchronization can work it out,then Telluride will advertise bgp routes to Alta.But I think that's not a true solution,cause AS100 is not a full mesh topoloogy,We must turn on synchronization to verify AS100's IGP has the route to outside AS100.

Why these routes are not synchronized?the next-hop is "192.168.1.221" and the routing table has the entry "O 192.168.1.220 [110/128] via 192.168.1.198, 00:45:50, Serial0";I even can ping "192.168.1.221" successfully from Telluride.It's very strangely!

[pkhatri · Accepted solution]

Hey Mate,

For synchronization to work, the BGP router-id of the router from which you are learning the routes has to be the same as the OSPF router-id.

Can you do the following on Vail:

router ospf 100

router-id

!

router bgp 100

bgp router-id

The router-id can be any unique IP address - it does not have to be an address of the interface on the router.

Hope that helps - pls rate the post if it does.

Paresh

[ilovebgp4]

hi!Pkhatri,though I haven't done the lab yet,I think you're right.Cause I found a topic about this in cisco's website : http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml , it says the same thing.

Do you know what's the story behind this rule?How did it came out?

This is the second time you solved my problem,do you still remember "how to set the lsa's p-bit in nssa area"?

[pkhatri]

Hey dude,

The 'rule' ensures that the IGP route is learned from the same router that the IBGP route is learned from - which is the entire premise of synchronisation. The following RFC specifies this rule - RFC1745

I think I do remember that earlier post from you :-)

Hope that helps - pls rate the post if it does.

Paresh