What is chosen first Admin distance or specific route?

[dvilardo]

Host is trying to get to 3.75.72.1.

If the router has a choice between 3.75.0.0/16 as an external BGP route and 3.75.72.0 as an internal BGP router - which is chosen?

[Georg Pauwen]

Hello,

the longest prefix is preferred, so in your case the iBGP route would be chosen.

You might want to check this document:

Route Selection in Cisco Routers

http://www.cisco.com/warp/public/105/21.html

HTH,

GP

[marikakis]

Most specific wins.

AD is considered when there is a need to break the tie between equally specific routes

supplied from sources of routing information that are considered different.

(e.g. eBGP is considered different from iBGP, by default).

M.

[mhussein]

This is tricky!

I think the external route is used (AD=20), although the internal route (AD=200) is more specific (/24 prefix).

However, If network 3.75.72.0 is configured on the router or redistributed via IGP, it would be preferred over the external route.

Reference:

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

ftp://ftp.rfc-editor.org/in-notes/rfc1772.txt

Regards,

Mustafa

[dvilardo]

I think your right. In fact in practice this is what I have seen. I had this exact scenerio and the more general External route was chosen contrary to what most would think.

[Richard Burts]

Can you post some information about this (especially the output of show ip route showing at least the two routes involved, and the evidence showing that traffic is forwarded to the more general route)? I would be very interested in seeing details showing that this happens.

HTH

Rick

[dvilardo]

I will if I can. In order to do so I have to schedule outage window - so I'm not sure when I can do it. But this particular network is very convoluted and I am learning not to be surprised by anything with this network. Its incorrectly designed and I have recently been given the task of engineering it. It's very daunting. In fact I also saw a 0.0.0.0 EIGRP route being chosen over a more spefic iBGP route.

The whole network is messed up - and a CCIE designed it. He had EIGRP redistributing into BGP and vice versa - instead of using network statements. I've given up on trying to understand it - I'm just trying to change it at this point.

[thisisshanky]

No doubt, forwarding decisions are based on longest match rule. Admin distance is used only in building the routing table. It is not used in selecting which route to use to forward the packet. Just like Rick suggested, any proof of the behaviour would help!!

[sdoremus33]

Providing these are the only two available routes EBGP has an administrative distance of 20 and IBGP 200,however if you want the IBGP to override EBGP I suggest you put the following command

router bgp 100

network 3.75.72.1 netmask (whatever your netmask is)

backdoor This should work)

Also clear the bgp table for this to work. BGP Neighbor must be reset for changes to take place

clear ip bgp*, which causes the TCP session between neighbors to be reset.

Hope this works