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BGP Terminology

Posted on 2011-10-24
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Last Modified: 2012-06-21
When I read about BGP, I get some confusion between:

-Route Reflector
-Next-Hop-Self
-Ebgp-Multihop

When should  I use one and not the other.
They could be different animals,but still don't have a clear view on each command.

I also want to understand this command:
Router2(Config-Router)# neighbor 10.1.1.1 Update-Source Loopback 0
Update-Source Loopback is referring to the loopback of Router2 or to the neighboor (in my case is Router1)?



thanks
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Question by:jskfan
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rochey2009 earned 2000 total points
ID: 37019168
Hi,

When using IBGP, there is the split horizon rule. A route learned from one IBGP neighbor will not be advertised to another IBGP neighbor. So you need to have IBGP peers fully meshed. This isn't very scalable so you can use route reflectors to reduce the number of IBGP peers.

Next hop self is used for example when the EBGP peer isn't reachable within you're IBGP network. If you weren't using next hop self, the other IBGP peers wouldn't be able to reach the next hop (which would remain as the EBGP peer) so they wouldn't install any of the routes because the next hop isn't reachable.

Ebgp defaults to a TTL of 1 hop. Ebgp-multihop is used to increase the number of hops.
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by:jskfan
ID: 37019278
the route reflector can be spokes too or is done at the hub only?
I got this one :Next hop self
Ebgp-multihop: not clear yet.

This one when you get a chance:
Router2(Config-Router)# neighbor 10.1.1.1 Update-Source Loopback 0
Update-Source Loopback is referring to the loopback of Router2 or to the neighboor (in my case is Router1)?
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by:rochey2009
rochey2009 earned 2000 total points
ID: 37019591
the route reflector is the hub and the route reflector clients are spoke.

update-source loopback is referring to the loopback address of Router 2.

EBGP peers default to 1 hop. The router sets TTL in it's BGP packets to 1.

In some cases, you may want the BGP packets to reach further than 1 one hop so you can increase the number of hops the BGP packet can travel using the Ebgp-multihop and specifying the TTL.

From Cisco:
To accept and attempt BGP connections to external peers residing on networks that are not directly connected, use the neighbor ebgp-multihop
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Author Closing Comment

by:jskfan
ID: 37029811
thanks
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