Information Technology Reference
In-Depth Information
iBGP in a Large Corporation
Figure 9-2
Asia
IGP
iBGP Core
EMEA
IGP
America
IGP
It is important to configure a full mesh between iBGP speakers within an AS to prevent routing
loops. When a BGP speaker receives an update from other BGP speakers in its own AS (iBGP),
the receiving BGP speaker does not redistribute that information to other iBGP peers. The
receiving BGP speaker redistributes that information to other BGP speakers outside of its AS.
External BGP
eBGP describes BGP peering between neighbors in different autonomous systems. As required
by the RFC, the eBGP peers share a common subnet. In Figure 9-3, all routers within AS 500
speak iBGP among each other, and they speak eBGP with the routers in other autonomous
systems.
eBGP Multihop
Although it is an exception to the norm, eBGP peers can be configured over several intervening
subnets by configuring eBGP multihop. eBGP Multihop is also used to peer with the loopback
address of a BGP neighbor that shares the same physical data-link segment. This is a proprietary
Cisco IOS feature. Static routing or an IGP protocol must provide connectivity between the
eBGP peers by using multihop. You only use multihop with eBGP peers; you do not use it with
iBGP. Figure 9-4 shows eBGP multihop peers.
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