User Manual

Enterasys X-Pedition User Reference Manual 207
BGP Configuration Examples
The gated.conf file for router X-Pedition2 is as follows:
IBGP Configuration Example
Connections between BGP speakers within the same AS are referred to as internal links. A peer in
the same AS is an internal peer. Internal BGP is commonly abbreviated IBGP; external BGP is
EBGP.
An AS that has two or more EBGP peers is referred to as a multihomed AS. A multihomed AS can
“transit” traffic between two ASs by advertising to one AS routes that it learned from the other AS.
To successfully provide transit services, all EBGP speakers in the transit AS must have a consistent
view of all of the routes reachable through their AS.
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 X-Pedition 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.
autonomoussystem 2 ;
routerid 10.0.0.2 ;
bgp yes {
group type external peeras 1
{
peer 10.0.0.1
;
};
};