BGP routing table

[fujin.huang]

Hi,

We know that BGP maintains a different routing table from IP, and there is procedure to compare the Attributes in order to install a route into IP routing table.

My question is that is there any criteria to be met in order to install a route into BGP ROUTING TABLE? which you can see by "show ip bgp".

I'm not able to find a cisco doc describe this, any idea?

Thanks for any idea.

Fujin

[thisisshanky]

Hi,

If the next hop of the BGP route is inaccessible the route is not taken into BGP table. This is one of criteria used to put a route first into BGP table. If you dont see that route in BGP table, it could most likely be because of this. Also in case of IBGP, if synchronization is not there inside AS between BGP and IGP, route might not be accepted in the BGP table. So in this case also you might find a route missing in BGP table.

shanky

[fujin.huang]

Hi, Shanky,

I believe what you said are true, but there must be something more, below is an example:

router A, B, C, D, E connected as circle, each router has a different AS. router A advertises a route 10.1.1.0/24, by right, router B should

have two paths to network 10.1.1.0/24, which are via router A and C, but in fact, router B only have one! try this out, if you don't believe.

Any further idea area appreciated.

Fujin

[m.haas]

Try the following link:

http://www.cisco.com/warp/public/459/25.shtml

This document describe the "BGP Best Path Selection"

and should answer your question.

[fujin.huang]

Hi, thank you, haas,

But I think you mis-understand my question, I'm talking about the BGP route advertisement and BGP routing table maintainenace, not IP routing table.

Anybody got any other opinion?

[ruwhite]

The BGP table (not routing table) is maintained using the BGP routing protocol... So, I'm not certain what you are asking. In general, BGP receives paths from peers, and uses the method described in the best path link to determine what the best path is, then installs those best paths, if eligible, in the routing table for the router to take action on.

:-)

Russ

[fujin.huang]

but are you able the explain the case that I describe above? Here I repeat it:

router A, B, C, D, E connected as circle, each router has a different AS. router A advertises a route 10.1.1.0/24, by right, router B should

have two paths to network 10.1.1.0/24, which are via router A and C, but in fact, router B only have one! try this out, if you don't believe.

Thanks for the help.

Fujin

[mazhar71]

This is test only

[ashok_boin]

Hi Fugin,

As your example said that Router B only have one path instead two paths, so what's your reasoning behind why that's happening to be? Are you sure that you are not using any policies for both inbound routes from Router A and C?

Regards...

-Ashok.

[velimirmkd]

Hi, first you have to put

no synchronization

in your BGP, second make sure that you have all the neighbor statements correct.

Velimir