Chapter 30. Border Gateway Protocol; Internal Routing Versus External Routing - Lenovo Flex System Fabric CN4093 Application Manual

Chapter 30. Border Gateway Protocol

Internal Routing Versus External Routing

© Copyright Lenovo 2015
Border Gateway Protocol (BGP) is an Internet protocol that enables routers on an
IPv4 network to share and advertise routing information with each other about the
segments of the IPv4 address space they can access within their network and with
routers on external networks. BGP allows you to decide what is the "best" route for
a packet to take from your network to a destination on another network rather than
simply setting a default route from your border router(s) to your upstream
provider(s). BGP is defined in RFC 1771.
CN4093 10Gb Converged Scalable Switches (CN4093s) can advertise their IP
interfaces and IPv4 addresses using BGP and take BGP feeds from as many as BGP
router peers. This allows more resilience and flexibility in balancing traffic from
the Internet.
Note: Lenovo N/OS 8.2 does not support IPv6 for BGP.
The following topics are discussed in this section:
 "Internal Routing Versus External Routing" on page 409
 "Forming BGP Peer Routers" on page 410
"What is a Route Map?" on page 411

 "Aggregating Routes" on page 414
"Redistributing Routes" on page 414

 "BGP Attributes" on page 415
"Selecting Route Paths in BGP" on page 416

 "BGP Failover Configuration" on page 417
 "Default Redistribution and Route Aggregation Example" on page 419
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.
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