bgp selection process

[rabeder]

hi,

i have an vpnv4 enabled bgp router which has the way to 10.59.16.0 /23 over two paths.

172.20.6.153 is a external neighbor

172.16.105.31 is a internal neighbor

!

Paths: (2 available, best #2, table whg)

Flag: 0x800

Advertised to peer-groups:

pe_linz_65052 pe_wien_65050 pe_innsbruck_65001 pe_graz_65005 pe_bregenz_65004

asbr_pe eigene_pe_active eigene_pe_passive pe_salzburg_65003 pe_rzb_65009

65004, imported path from 65056:543:10.59.16.0/23

172.20.6.153 from 172.20.6.153 (172.16.72.1)

Origin incomplete, localpref 100, valid, external

Extended Community: RT:65056:530 RT:65056:543,

mpls labels in/out 116/1073

Local, (Received from a RR-client)

172.16.105.31 (metric 29) from 172.16.105.31 (172.16.105.31)

Origin incomplete, metric 0, localpref 100, valid, internal, best

Extended Community: RT:65056:500,

mpls labels in/out 116/104

BGP routing table entry for 65056:543:10.59.16.0/23, version 244402

!

why does the router choose the way over 172.16.105.31

thanks for answer !

[sourabhagarwal]

you ned to know about BGP route selection process for this. check this URL to get that information.

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

If you see the URL, you will observe that in your case weight, local preference, AS_PATH attribute is same for both paths, hence they can't decide to select the route.

Due to this, origin atribute was selection crieria for selecting the best route. lowest origin type will be preferred route.

IGP is lower than Exterior Gateway Protocol (EGP), and EGP is lower than INCOMPLETE.

In your case as 172.16.105.31 is IGP while 172.20.6.153 is EGP neighbor, hence router choose the way over 172.16.105.31.

hope it helps .. rate if it does ..

[network.king]

Hi

I think both the routes have the same origin "incomplete " . Its all the AS path which comes into decision here .

As weight , local pref are same , then comes ur AS path and second one doesnt have one while the first one will have a AS number.

Hope this helps

regards

vanesh k

[ranjanibharadwaj]

The origin code is incomplete for both routes. Seems like they both are redistributed routes. I am not too familiar with configuration of communities, but to me it looks like the decision is being made upon the AS_Path. Please comment. Thank you.

[sourabhagarwal]

I agree, origin is incomplete for both paths.

I feel metric (MED) is deciding factor for the route selection in this case. path with the lowest multi-exit discriminator (MED) will be preferred.

MED (metric) values are locally significant to an AS and are set according to the local policy. default value is 100. As we can see metric for path via 172.16.105.31 is 29 and for path via 172.20.6.153 is default, so path via 172.16.105.31 is preferred.

[rabeder]

hi to everyone,

thanks for any answer, but:

as i can remember from bgp training, the selection prozess is:

next hop reachable

weight

local preference

directly conncted routes

AS-Path

origin-attribute

MED

as u can see first is AS-Path then later MED.

so in my case 172.16.105.31 is a iBGP neighbor and

172.20.6.153 is eBGP - so here we have an AS-Path and for 172.16.105.31 not.

is this the reason why the way goes over 105.31 ?

thanks and greetings from austria

[sourabhagarwal]

can you share output of "show ip bgp 10.59.16.0" from the BGP router?

[network.king · Accepted solution]

Hi

Yes you are correct , Its all the AS path which has taken the decision of best path.

regards

vanesh k