Internal Routing Versus External Routing - Lenovo RackSwitch G8264 Application Manual

Internal Routing Versus External Routing

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G8264 Application Guide for ENOS 8.4
To ensure effective processing of network traffic, every router on your network
needs to know how to send a packet (directly or indirectly) to any other
location/destination in your network. This is referred to as internal routing and can
be done with static routes or using active, internal dynamic routing protocols, such
as RIP, RIPv2, and OSPF.
Static routes must have a higher degree of precedence than dynamic routing
protocols. If the destination route is not in the route cache, the packets are
forwarded to the default gateway which may be incorrect if a dynamic routing
protocol is enabled.
It is also useful to tell routers outside your network (upstream providers or peers)
about the routes you can access in your network. External networks (those outside
your own) that are under the same administrative control are referred to as
autonomous systems (AS). Sharing of routing information between autonomous
systems is known as external routing.
External BGP (eBGP) is used to exchange routes between different autonomous
systems whereas internal BGP (iBGP) is used to exchange routes within the same
autonomous system. An iBGP is a type of internal routing protocol you can use to
do active routing inside your network. It also carries AS path information, which is
important when you are an ISP or doing BGP transit.
The iBGP peers have to maintain reciprocal sessions to every other iBGP router in
the same AS (in a full‐mesh manner) to propagate route information throughout
the AS. If the iBGP session shown between the two routers in AS 20 was not
present (as indicated in Figure
50, and the bottom router would not learn the route to AS 11, even though the two
AS 20 routers are connected via the RackSwitch G8264.
Figure 43. iBGP and eBGP
When there are many iBGP peers, having a full‐mesh configuration results in large
number of sessions between the iBGP peers. In such situations, configuring a route
reflector eliminates the full‐mesh configuration requirement, prevents route
propagation loops, and provides better scalability to the peers. For details, see
"Route Reflector" on page
43), the top router would not learn the route to AS
Internet
Internet
533.