default local pref with iBGP mesh

[from88]

plese look at:

http://imageshack.us/photo/my-images/810/localpref.jpg/ prefix 183.0.0.0/8.

Why it chooses the 192.168.1.6 as the best path. I know the best path algorithm and so on, but the question in why the best path isn't the 10.100.1.1 with local pref 100. Just because it's default local pref it just dont play a role in route selection ? Thanks

[skarthic]

Yes. The default Local Pref is 100 and may not be considered here. From the given o/p, looks like the EBGP path is getting preferred over an IBGP path since the weight,metric and the Local pref are the same

.

Regards

Subramani.

[from88]

It's from Wendell's book. Why it's not considered here ? If i would change for that route the local pref for example to 101 or 99 it would be considered ?

[skarthic]

Yes. By default all the routes will have a local preference of 100(even the external route).

The "show ip bgp " will give further detail.

However, changing the local preference for one of the paths will influence your routing decision, with the path with the higher Local pref getting preferred.

[from88]

thanks, after reading about four different sources finaly found:

http://fengnet.com/book/Cisco.IOS.Cookbook.2nd/I_0596527225_CHP_9_SECT_8.html

Router2#show ip bgp
BGP table version is 4, local router ID is 172.18.5.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
* i10.0.0.0         172.18.5.2                    100      0 65510 65531 i
*>                  192.168.2.5                            0 65520 65531 i
Router2# 

This router learned the second route through eBGP, so it doesn't have a Local Preference value. The router treats any missing Local Preference values as if they had the default value of 100. Note that although BGP has a route selection rule that prefers eBGP routes to iBGP routes, this rule is consulted after the Local Preference is, but for now the Local Preference values are equal. So, all other things being equal, this router prefers to use the more direct route that it learned itself, which is indicated by the ">" character at the beginning of the line.

[ashok_boin]

Yes, in this specific case, it's due to eBGP win over iBGP route as other path attributes would be same in Bestpath selection algorithm.

You may try with increasing local preference for iBGP route above 100 and look at the result.

Regards..

-Ashok.