BGP Path Selection

[Professor_Pickles]

Greetings, 

Long time reader first time poster, my question for the group is that of a could you and if you can how do you this? Below is a quick abstract illustration of what I'm working with, so:

1. I need to reach the host AS60 from AS10
2. AS60 has manipulated best path selection via as-prepend for the AS60 network in order to favor the AS30 link over AS40/50.(AS50 being the default best path) 
1. So, the path selection would be from my AS10 host> AS10>AS20>AS30>AS60> AS60 Host
3. For performance reasons I need to be able to use send my packets down the AS50 link to AS60 
1. Path would be my AS10 host> AS10>AS20>AS50>AS60> AS60 Host

That all said, AS60 will not be able to change their as-prepend, so my question is there anyway I can work with my AS20 carrier in this case, to move my packets down AS50 rather than AS30?

Thanks, Phil

[Ulrich Nguelet]

This is the output i got from CCIE TShoot that i am trying to solve. But it looks like something wrong and I cannot explain. I went to a lot of document explaining bgp path selection and I still cannot explain why bgp chooses the ‘internal’ route over the ‘external’ one. even if I set the weight higher.

 Can someone help please.

R18#sh bgp vpnv4 u all 180.10.123.0/25
BGP routing table entry for 65000:65000:180.10.123.0/25, version 26
Paths: (2 available, best #2, table SiteA)
Advertised to update-groups:
23
Refresh Epoch 1
65004
180.10.186.0 (via vrf SiteA) from 180.10.186.0 (172.16.1.6)
Origin incomplete, metric 0, localpref 100, weight 1000, valid, external
Extended Community: RT:65000:65000
rx pathid: 0, tx pathid: 0
Refresh Epoch 1
Local
199.8.2.100 (metric 2) (via default) from 199.8.2.100 (199.8.2.100)
Origin incomplete, metric 117760, localpref 100, valid, internal, best
Extended Community: RT:65000:65000 Cost:pre-bestpath:128:117760
0×8800:32768:0 0×8801:56:5632 0×8802:65283:2560 0×8803:65281:1400
0×8806:0:2886729991
mpls labels in/out nolabel/16
rx pathid: 0, tx pathid: 0×0

[Jon Marshall]

Phil

Sorry I may not have answered your specific question.

Whether you need to move to a /24 depends on what your carrier will accept from you so I can't really say for sure to be honest.

Jon