[isp-bgp] Re: BGP administrative distance question

[Iljitsch van Beijnum <iljitsch@...>]

On 7-mei-04, at 17:41, Thanh_Mai@... wrote:

> This is in reference to IBGP and EBGP distances only, so If I set EBGP 
> to
> 20 and IBGP to 200, I will always preferr EBGP regardless of AS-PATH, 
> etc.
> NO IGP routes involved.
Actually it's the default for eBGP routes to overrule any IGP routes.

If what you want to do is prefer eBGP routes over iBGP ones: it doesn't 
work that way. The administrative distance is used to select the best 
route when more than one routing protocol has a route for a 
destination. This happens AFTER each routing protocol has selected the 
best route for inclusion in the master routing table ("show ip route") 
as per the algorithm pertinent to that protocol. (Least number of hops 
for RIP, lowest cost for OSPF, the list of 15 or so rules for BGP...)

In other words: BGP first selects a route for inclusion in the routing 
table using the regular BGP route selection algorithm (or "path 
selection algorithm" if you prefer). Then, if this route happens to be 
learned over eBGP, it gets an administrative of 20. If not, it gets a 
distance of 200.

I haven't tried this, but you should be able to reach the desired 
effect by using a "match route-type external" clause in a route map 
that increases the local preference (or weight).

Imagining what kind of bad stuff will happen if you prefer internal 
routes over external ones instead is left as an exercise for the 
reader.

--
Iljitsch van Beijnum  -  http://www.bgpexpert.com/  (updated: Apr 24, 
23:25:01)







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