Ah, after checking up on this I see where I was confused.
Synchronization does not specifically refer to the behavior we were
talking about. I thought that synchronization meant that the next-hop
had to be in the routing table before a prefix could be moved from the
BGP table to the routing table. That's not quite correct. I'll quote a
portion of Halabi:
"The BGP rule states that a BGP router should not advertise to external
neighbors destinations learned from iBGP neighbors unless those
destinations are also known via an IGP. This is known as
synchronization. If a router knows about these destinations via an IGP,
it assumes that the route has already been propagated inside the AS, and
internal reachability is ensured."
Thanks for pointing this out, Peter. Someone on the list recently
pointed out that BGP synchronization and ip classless seem to be in the
class of misunderstanding. Just when you think you really understand how
it operates, you realize you have it wrong. <g> I think I have it now!
Maybe...
John
>>> "Peter Van Oene" <[EMAIL PROTECTED]> 4/2/01 7:03:16 AM >>>
Synch is an issue that gets way too much attention in my opinion. It's
not used at all. It's a legacy feature that is meaningless in todays'
networks.
What John describes below, the fact that IBGP routers will no post
routes unless they have reachability to the Next_Hop is not a
synchronization issue, rather it is a fundamental function of BGP. If
routers started posting routes that they have no hope of delivering
traffic to, things would get pretty messy pretty fast. Thankfully,
there are no nobs to turn this _behavior_ off :)
*********** REPLY SEPARATOR ***********
On 4/1/2001 at 4:58 PM John Neiberger wrote:
>When an eBGP neighbor forwards routing information to another eBGP
>neighbor,
>it changes the next hop to itself. When an iBGP neighbors exchange
>information they do not, by default, change the next hop. This is
where
>the
>synchronization rule comes in.
>
>An iBGP neighbor will not be able to use a route if it does not have
a
>valid
>route to the next hop in its IGP. Having synchronization turned on
is
>often
>unnecessary, so most people turn it off. You still have a problem,
>though:
>the receiving iBGP neighbor still might not know how to reach the next
hop
>for any of the routes in its BGP table. To solve this, on your iBGP
peers
>use the next-hop-self command. Since the peers already know how to
reach
>each other, this solves your problem.
>
>I hope that helps, and I hope I haven't mischaracterized the issue.
I
>haven't really thought through all of this in a while so I may have
some
>details wrong.
>
>If you really want to understand this stuff, pick up a copy of
Internet
>Routing Architectures (2nd Ed.) by Sam Halabi.
>
>Another book I really liked is short but sweet. It's BGP4:
Interdomain
>Routing in the Internet (or something close to that.) It's very short
but
>it's an excellent resource. Perhaps you should read that first and
then
>read Halabi.
>
>Or you could also get a subscription to Certification Zone and read
>Howard's
>papers on BGP, they're quite excellent.
>
>HTH,
>John
>
>> I'm really confused about the how Next-hop attribute works for IBGP
and
>> EBGP. Can somebody please shed some light on this. Any tips or
help
>> would be greatly appreciated.
>>
>> Regards,
>> Hunt
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