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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