BGP route not being advertised

[adil.javid]

Hello,

I have a route that I am trying to advertise to an external provider via BGP, but with no success.

- This is an EBGP connection;

- I have the route in my IGP table (OSFP);

- I have given the network statement in BGP to advertise the route;

- I have modified the prefix-list to include the new route to advertise.

However, even after all these changes above I cannot see the route being advertised to the provider.

Does anyone know what could be causing this issue please???

Thank in advance.

[adil.javid]

Hi,

Thanks for everones inputs.

Last night we tried a clear BGP soft and both ends and the route started to be propogated.

Thanks to all.

[shiva_ial]

may be synchronisation issue

your route will not be advertised to service

provider(another EBGP)until all IGP routers are aware of the route (all routers have same kind of information about the route)

so to override this

use no synchronisation command

experts can correct me if i am wrong, gives a learning edge as well

[Kevin Dorrell]

You are right that if he is using synchronisation, the IGP should be aware of the route. Also that switching off synchronisation will remove that restriction. But I think that in this case the router does have a route. (Although Milan's issue about the mask still needs to be investigated.)

You might be interested to know that in the case of OSPF and BGP, synchronisation has one further special constraint. With most IGPs, the IGP just needs to know a route to the prefix, and BGP will be allowed to use it. But with OSPF, the originating router id must be the same as the BGP router id.

We could see all this information if the original poster could post a specific show ip bgp 211.14.0.0 for the route in question.

Kevin Dorrell

Luxembourg

[marikakis]

Hello,

I think the discrepancy in the subnet mask did not matter after all, because the prefix filter allowed "more" networks than the /24 implied by the lack of subnet mask in the network bgp command.

In any case, the exact intended mask should have been known and used, rather than depending on luck in order to see a 211.14.0.0 route being advertised, without paying attention to the mask that accompanies it.

The bgp implied a /24 network origination, the IGP route knew about a /16 and filter allowed a /16. So, I guess that the route that is actually being advertised after all is a /24.

Anyway, if you advertise less networks than you are assigned, problem will be in your network with some parts of it at some point being unreachable. Advertising more that you are assigned and your bgp peers finding it out is the humiliating scenario.

Kind Regards,

M.

[milan.kulik]

Hi,

IMHO, an exact match beween BGP network command and a routing table is necessary to advertise a prefix.

And also

ip prefix-list SG-To-Thomson seq 70 permit 211.14.0.0/16

does not permit /24 if LE or GE options are not used.

I agree with Kevin

show ip bgp 211.14.0.0/16 longer

would help to see what's going on there exactly.

BR,

Milan

[marikakis]

"An exact route in the routing table is required for a network statement with a mask in order for it to be installed into a BGP table." from http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800945ff.shtml#topic4

Network statement did not have a mask as you have pointed out earlier in this discussion.

Wierd things can also happen with auto-summary enabled:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800945ff.shtml#topic1

Do you have any other documentation pointer that says something different?

You are right about the prefix-list, I have not looked at it thoroughly.

If we had all the config (that prefix-list could have other entries that allow the prefix to be advertised)

or some feedback about what actually changed besides a soft clear of the BGP session, perhaps we could solve the mystery.

Kind Regards,

M.

[Kevin Dorrell]

Welcome back Marikakis! Looking forward to reading your postings once again.

Kevin Dorrell

Luxembourg

[marikakis]

Hello Kevin! I always consider this forum great and great you are all of you people that spend so much of your time helping others! I hope I will soon have more time to catch up with your postings as well!

[milan.kulik]

Hi,

AFAIK, the BGP network ... command without mask opion is treated classfull, i.e.

network 211.14.0.0

should be the same as

network 211.14.0.0 mask 255.255.255.0

And even with auto-summary enabled, the prefix advertised would also be classful, i.e. 211.14.0.0/24.

I agree without knowing more config details and sh ip bgp 211.14.0.0/16 longer output we can only guess what's really happenning there.

BR,

Milan