ECS4660-28F_Management Guide-R03

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1811

Table Of Contents

HAPTER

| IP Routing Commands

Border Gateway Protocol (BGPv4)

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Figure 555: Connections for Internal and External BGP

External BGP – eBGP interconnects different ASs through border routers,

or eBGP peers. These peering routers are commonly connected over a WAN

link using a single physical path. Alternatively, multiple eBGP peer

connections may be used to provide redundancy or load balancing. Distinct

BGP sessions are used between redundancy peers to ensure that if one

session fails, another will take over.

BGP uses the AS path attribute to record the ASs that must be followed to

reach the prefix for a network aggregate. When a prefix is announced to an

eBGP peer, the local AS number is prepended to the AS path. This prevents

routing loops by rejecting any prefix announcements that include the local

AS number in the AS path. These announcements are also used by eBGP in

the best path selection process.

eBGP speakers, can communicate with other external peers or with iBGP

peers. A BGP speaker can determine if it communicating with an external

or internal peer by comparing the AS number sent in OPEN messages by a

peer with that of its own internal value. If it matches, then this neighbor is

an iBGP speaker, and if it does not, then it is an eBGP speaker. An eBGP

speaker can advertise prefixes it has learned from another eBGP speaker to

a neighboring iBGP speaker; and it can also advertise prefixes it has

learned from an iBGP speaker to an eBGP speaker.

Internal BGP – In contrast to eBGP peers which have different AS

numbers, iBGP peers are configured with the same AS number. All iBGP

peers within the same AS should be connected to one another in a full-

mesh connection (except when using route reflection). When a prefix is

announced from one iBGP peer to another, the AS path is not changed.

Since all iBGP peers are fully meshed, they will have the same information

in their BGP table, unless routing policies have been modified for some of

the peers.

When a iBGP peer receives a prefix announcement, it uses the best path

selection algorithm to see if the received announcement is the best path

for that prefix. If it is, the peer inserts this route into its routing table, and

announces it to all of its peers, both iBGP and eBGP. If it is not the best

Router

AS200

Router

AS100

Router

AS300

eBGP

eBGPeBGP

eBGP

iBGP