Lenovo ThinkSystem NE2552E, Application Manual

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NE2552E Application Guide for ENOS 8.4
Internal Routing Versus External Routing
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 should have a higher degree of precedence than dynamic routing
protocols. If the destination route is not in the route cache, then 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) in order 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 37 ), the top router would not learn the route
to AS 50, and the bottom router would not learn the route to AS 11, even though
the two AS 20 routers are connected via the Flex System and the Application
Switch.
Figure 37.
iBGP and eBGP
Typically, an AS has one or more border routers —peer routers that exchange routes
with other ASs—and an internal routing scheme that enables routers in that AS to
reach every other router and destination within that AS. When you advertise routes
to border routers on other autonomous systems, you are effectively committing to
carry data to the IPv4 space represented in the route being advertised. For
example, if you advertise 192.204.4.0/24, you are declaring that if another router
sends you data destined for any address in 192.204.4.0/24, you know how to carry
that data to its destination.
AS 11
Internet eBGP
iBGP
ISP A
AS 50
ISP B
AS 20
Application
Switch