Riverstone Networks WICT1-12 - Manual (Page 275)

Riverstone Networks RS Switch Router User Guide Release 8.0 16-17
BGP Configuration Guide BGP Configuration Examples
Multihomed transit ASs can use IBGP between EBGP-speaking routers in the AS to synchronize their routing tables.
IBGP requires a full-mesh configuration; all EBGP speaking routers must have an IBGP peering session with every
other EBGP speaking router in the AS.
An IGP, like OSPF, could possibly be used instead of IBGP to exchange routing information between EBGP speakers
within an AS. However, injecting full Internet routes (50,000+ routes) into an IGP puts an expensive burden on the
IGP routers. Additionally, IGPs cannot communicate all of the BGP attributes for a given route. It is, therefore,
recommended that an IGP not be used to propagate full Internet routes between EBGP speakers. IBGP should be used
instead.
IBGP Routing Group Example
An IBGP routing group uses the routes of an interior protocol to resolve forwarding addresses. An IBGP routing group
will determine the immediate next hops for routes by using the next hop received with a route from a peer as a
forwarding address, and using this to look up an immediate next hop in an IGP’s routes. Such groups support distant
peers, but need to be informed of the IGP whose routes they are using to determine immediate next hops. This
implementation comes closest to the IBGP implementation of other router vendors.
You should use the IBGP routing group as the mechanism to configure the RS for IBGP. If the peers are directly
connected, then IBGP using group-type Internal can also be used. Note that for running IBGP using group-type routing
you must run an IGP such as OSPF to resolve the next hops that come with external routes. You could also use protocol
any
so that all protocols are eligible to resolve the BGP forwarding address.
Figure 16-3 shows a sample BGP configuration that uses the routing group type.
Figure 16-3 Sample IBGP configuration (routing group type)
R
6
R
1
Cisco
R
4
lo0 172.23.1.25/30
10.12.1.6/30
10.12.1.5/30
172.23.1.10/30
172.23.1.9/30 172.23.1.6/30
lo0 172.23.1.26/30
172.23.1.5/30
10.12.1.2/30
10.12.1.1/30
AS-64801
IBGP
OSPF