From: ccie2be (ccie2be@nyc.rr.com)
Date: Tue Apr 08 2003 - 20:43:07 GMT-3
Hi Barry,
I happen to be reviewing BGP right now myself and have wondered the same
thing a number of times (I seem to always forget the rules). However, I can
tell you this. If you review the Halabi book, he goes over 2 scenario's
where you must use the Next-Hop-Self attribute.
Basically, Halabi describes 2 scenarios:
1) When the link between eBGP peers isn't known by the IGP. (To me, this is
the easiest & most intuitive situation) Why? Imagine this.
Rtr A & B are in AS 1 and are running some IGP, say RIP along with iBGP.
Rtr C is in AS 2 and has a link to rtr B. When rtr C advertises routes to
rtr B, it behaves just like any Distance Vector router and tells rtr B, it
can reach these nets by sending pkts to me at addr abcd and rtr B knows
about addr abcd b/c its directly connected to it. So, rtr B knows how to
address pkts going to nets learned from rtr C. Now, what happens when rtr B
advertises those routes it learned from rtr C to rtr A via iBGP? The answer
depends upon whether RIP knows about the link between rtr B & C. By def,
routes learned via eBGP are advertised to iBGP peers with the Next-HOP-Addr
unchanged. So, if RIP knows how to reach rtr C, no problem cause then rtr A
will also know how to reach rtr C. But, what if RIP doesn't know about the
link betw rtr B & C? Then, how can rtr A know how to reach rtr C. It
CAN'T!!! To resolve this problem there are 2 solutions:
1) Make sure the IGP knows about the link betw the eBGP peers or
2) On rtr B, use the nei xxxx next-hop-self command
Which is better? They both work equally well, however, with the 1st method,
you can ping rtr C from within your AS.
2) The 2nd scenario where you need to use Next-Hop-Self. In the 1st
scenario, the assumption was that the links betw rtr A & B and betw rtr B &
C were on different subnets. But, suppose they're not. Suppose rtr B is
the hub rtr of a multipoint f/r network which is partially meshed. Then,
you must configure Next-Hop-Self on rtr B. This is important to remember.
If you apply the logic of the 1st scenario to this scenario, you're screwed.
Yes, in this scenario, the IGP knows about the link betw routers B & C, so
you might be tempted to think, "OK, I don't have to configure
Next-Hop-Self". But, IT WON'T WORK!!!!
Hope this helps. Jim
PS: To the Group Study community -- If I've mistated anything, please
correct me immediately
----- Original Message -----
From: "Barry Nolan" <bnolan@euro.banta.com>
To: <ccielab@groupstudy.com>
Sent: Tuesday, April 08, 2003 5:17 AM
Subject: OT: Next-hop-self
> Hi All,
> Could anyone advise a good rule of thumb for the use and non use
> next-hop-self BGP command ?
> Something along the lines of ; use if x,y,z but not a,b,c
>
> Thank You,
> Barry Nolan.
>
>
>
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