BGP Questions

Unanswered Question
Nov 11th, 2009

Folks:

I have a few BGP questions:

1.) What does it mean when a BGP speaker advertises a prefix without a prefix-length attached?

sh ip bgp nei 64.13.96.84 advertised-routes

*> 144.223.0.0 144.223.0.93 0 0 1239 i

144.223.0.92/30 is the eBGP subnet that this router uses with the ISP.

2.) Why does the BGP table show a /16 advertisement for 144.223.0.0 from the ISP (144.223.0.93), yet the IP routing table shows that it is a directly connected route?

sh ip bgp 144.223.0.93

BGP routing table entry for 144.223.0.0/16, version 9863746

Paths: (1 available, best #1, table default)

Advertised to update-groups:

1

1239

144.223.0.93 from 144.223.0.93 (144.228.241.4)

Origin IGP, metric 0, localpref 100, valid, external, best

sh ip ro 144.223.0.93

Routing entry for 144.223.0.92/30

Known via "connected", distance 0, metric 0 (connected, via interface)

Routing Descriptor Blocks:

* directly connected, via TenGigabitEthernet9/1

Route metric is 0, traffic share count is 1

3.) The next hop self command is used between this bgp speaker and its iBGP neighbor, so why does the next hop show as 144.223.0.93, instead of the loopback interface? take note, that the receiving router does see the other routers loopback interface as the next hop.

sh ip bgp nei 64.13.96.84 advertised-routes

BGP table version is 12174848, local router ID is 64.13.96.85

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 0.0.0.0 144.223.0.93 0 1239 i

*> 2.0.0.0/16 144.223.0.93 0 1239 6453 12654 i

*> 2.1.0.0/21 144.223.0.93 0 1239 6453 12654 i

Its as if the next hop self command acts as a policy that is applied after the fact.....

Thanks

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Jon Marshall Wed, 11/11/2009 - 19:52

Victor

1) & 2) - you have got "auto-summary" turned on in your config so BGP is summarising at classful boundaries ie. class A, B & C networks.

Edit - sorry forgot about 3).

3) This is what i would expect to see.

You are looking at the local RIB table. The next-hop is changed when the route is advertised to it's neighbor.

Jon

Edison Ortiz Wed, 11/11/2009 - 20:24

2) The ISP is advertising a summary route for 144.223.0.0 and the reason you don't see the 92/30 subnet is because it's not advertised on BGP.

When you issue the 'show ip bgp' command for that route, it will choose the closest route match on BGP - on this case 144.223.0.0/16.

Advertise 92/30 in BGP via network statement or redistribution - this will provide the expected result.

Edison.

lamav Wed, 11/11/2009 - 20:44

Edison:

Makes perfect sense. And I actually realized the answer to my question a bit after posting it. I guess sometimes articulating a question - either verbally or in writing -- forces your mind to jog a bit and draw the conclusion you have to.

The conclusion I draw was that the ISP is advertising a /16 through eBGP and that router was passing it on to its iBGP neighbor, exactly as expected.

I was wrongly thinking that the /30 subnet (carved out of the /16) would be advertised through iBGP by that router, too, and that it would advertise itself as the next hop because it is directly connected after all. But thats where I forgot that, because the router is originating the route, to advertise it through BGP, you must tell it to by using the network command or redistribution...in other words, it is not like th other eBGP routes - it was not learned, it is originating it.

right?

Jon:

Auto summary is disabled on both routers. Every other advertisement I get from the ISP has a prefix length attached, except for a few that begin with 144.

Still dont see why....

As for question 3, I guess thats just the behavior....

Mohamed Sobair Thu, 11/12/2009 - 00:44

Hi

EDIT:

2)Network 144.223.0.0 must be redistributed prefix into BGP as the (Auto-summary) command has only affect on redistributed prefixes.

HTH

Mohamed

lamav Thu, 11/12/2009 - 04:57

Hi:

Auto summarization is disbaled on both routers,

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