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New Member

Need help about BGP route advertisement

Dear Sir,

I am getting some odd type of output of the "show ip bgp A.B.C.D" for my routed C class.I am pasting it below. please let me know if there is anything wrong as the AS 17557 (AS of our service provider) has many entries at some points.Please help me .

route-server.exodus.net>sh ip bgp 202.165.245.0

BGP routing table entry for 202.165.245.0/24, version 12021612

Paths: (16 available, best #2)

Not advertised to any peer

701 1 7473 17557

209.1.220.194 from 209.1.220.194 (209.1.220.194)

Origin IGP, localpref 1000, valid, internal

701 1 7473 17557

209.1.40.51 from 209.1.40.51 (209.1.40.51)

Origin IGP, localpref 1000, valid, internal, best

3561 1 7473 17557

209.1.220.102 from 209.1.220.102 (209.1.220.102)

Origin IGP, localpref 1000, valid, internal

3561 1 7473 17557

209.1.40.148 from 209.1.40.148 (209.1.40.148)

Origin IGP, localpref 1000, valid, internal

3561 1 7473 17557

209.1.220.174 from 209.1.220.174 (209.1.220.174)

Origin IGP, localpref 1000, valid, internal

3561 1 7473 17557

209.1.220.242 from 209.1.220.242 (209.1.220.242)

Origin IGP, localpref 1000, valid, internal

3561 1 7473 17557

209.1.220.9 from 209.1.220.9 (209.1.220.9)

Origin IGP, localpref 1000, valid, internal

3561 1 7473 17557

209.1.40.72 from 209.1.40.72 (209.1.40.72)

Origin IGP, localpref 1000, valid, internal

5400 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557

209.1.220.7 from 209.1.220.7 (209.1.220.7)

Origin IGP, localpref 1000, valid, internal

7018 5400 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557

209.1.220.134 from 209.1.220.134 (209.1.220.134)

Origin IGP, localpref 1000, valid, internal

7018 5400 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557

209.1.220.156 from 209.1.220.156 (209.1.220.156)

Origin IGP, localpref 1000, valid, internal

7018 5400 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557

209.1.220.66 from 209.1.220.66 (209.1.220.66)

Origin IGP, localpref 1000, valid, internal

7018 5400 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557

209.1.220.133 from 209.1.220.133 (209.1.220.133)

Origin IGP, localpref 1000, valid, internal

7018 5400 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557

209.1.220.126 from 209.1.220.126 (209.1.220.126)

Origin IGP, localpref 1000, valid, internal

7018 5400 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557 17557

209.1.220.41 from 209.1.220.41 (209.1.220.41)

Origin IGP, localpref 1000, valid, internal

3561 1 7473 17557

209.1.40.141 from 209.1.40.141 (209.1.40.141)

Origin IGP, localpref 1000, valid, internal

4 REPLIES
Silver

Re: Need help about BGP route advertisement

Judging by the data you posted it looks like your ISP is using a BGP feature called 'as-path prepend'. Your ISP has multiple uptream providers, AS 7473 and AS 5400. The BGP routes advertised to AS 5400 have their AS#17557 prepended may times in the as-path list. This is insure that the routers in the internet do not choose the path through AS 5400 so long as the path through AS 7473 is good.

This would make sense if your ISP's link to AS 5400 is much slower than their link to AS 7473, such DS3 vs. T1, among other possible scenarios/policies.

Regards,

Brad

Cisco Employee

Re: Need help about BGP route advertisement

there was also a bug related to strange output like this. I can't find the bug id anymore but I would suggest to run a latest version and see if the problem is still there.

New Member

Re: Need help about BGP route advertisement

1.AS-Path prepend is in use. The purpose of prepending is to make a route less desirable.

2.In this instance, routers will route packets destined to network AS17557 via AS7473(provided link is up) as it has fewer hop counts to the network compared with AS5400.

regards,

Victor

New Member

Re: Need help about BGP route advertisement

Don't worry, nothing wrong to your ISP announcing your small IP block. they are doing "prepend As-path" to choose the route for u between their two upstream providers. Since you took the exodus.USA as the reference, you just saw your IP block is being announced by many BGP routers there.

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