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Cisco CCNP Certification: Using The BGP Command "Update-Source" |
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When you start preparing for your CCNP exam, particularly
the BSCI exam, you're introduced to Border Gateway Protocol
(BGP) configurations. BGP is unlike any protocol you learned
during your CCNA studies, and even the similarities are a
little bit different!
BGP forms neighbor relationships, much like EIGRP and OSPF
do. The interesting thing with BGP is that potential neighbors,
or "peers", do not need to be directly connected
and can use their loopback interfaces to form the peer relationships.
It may well be to your advantage to use loopbacks to form
peer relationships rather than the actual interface facing
the potential neighbor. This can be done because BGP uses
static neighbor statements rather than any kind of dynamic
neighbor discovery process.
Consider a router that has two paths to a BGP speaker. The
interfaces are numbered like this:
Router1: Serial0, 172.1.1.1 /24, Serial2, 179.1.1.1 /24,
loopback0, 1.1.1.1 /32.
Router2: Serial0, 172.1.1.2/24, Serial2 179.1.1.2/24, loopback0,
2.2.2.2 /32.
We could configure Router1 like this:
router bgp 200
neighbor 172.1.1.2 remote-as 200
In this case, BGP would automatically use 172.1.1.1 as the
source for the TCP connection that has to be set up with the
neighbor before updates can be exchanged; this address is
known as the best local address. However, if the remote peer's
serial0 interface is shut down or goes down for another reason,
the peer relationship would be lost even though Router2 is
still available.
Instead of using one of the physical interfaces, we can use
the loopbacks on each router to establish the TCP-based peer
connection. The configurations would look like this:
Router1:
router bgp 200
neighbor 2.2.2.2 remote-as 200
neighbor 2.2.2.2 update-source loopback0
Router2:
router bgp 200
neighbor 1.1.1.1 remote-as 200
neighbor 1.1.1.1 update-source loopback0
In this case, losing one of the physical connections does
not necessarily mean the BGP peering is lost; as long as the
routers have a valid path to each other's loopback addresses,
the BGP peer relationship will stay in place. And better yet,
we avoid the dreaded “single point of failure”!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNP and CCNA tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, just visit the website!
You can also get FREE CCNA and CCNP exam questions every day!
Pass the CCNP exam with The Bryant Advantage!
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