Chapter 21. Border Gateway Protocol; Internal Routing Versus External Routing - IBM RackSwitch G8000 Application Manual

Chapter 21. Border Gateway Protocol

Internal Routing Versus External Routing

© Copyright IBM Corp. 2011
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.
RackSwitch G8000es can advertise their IP interfaces and IPv4 addresses using
BGP and take BGP feeds from as many as 16 BGP router peers. This allows more
resilience and flexibility in balancing traffic from the Internet.
Note: IBM Networking OS 6.8 does not support IPv6 for BGP.
The following topics are discussed in this section:
• "Internal Routing Versus External Routing" on page 245
• "Forming BGP Peer Routers" on page 246
• "Loopback Interfaces" on page 247
• "What is a Route Map?" on page 247
• "Aggregating Routes" on page 251
• "Redistributing Routes" on page 251
• "BGP Attributes" on page 251
• "Selecting Route Paths in BGP" on page 252
• "BGP Failover Configuration" on page 253
• "Default Redistribution and Route Aggregation Example" on page 254
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.
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