Internal and External BGP

We have already used the terms Internal BGP and External BGP (IBGP/EBGP) a few times leading up to this point. It’s time to explore what this terminology signifies. For the most part, BGP operation is the same when operating internally to an AS versus externally to a remote AS, but Table 7-1 summarizes the key differences.

Table 7-1. IBGP and EBGP

Characteristic/attribute

IBGP

EBGP

Local AS added to AS path

No

Yes

Next hop overwritten

No

Yes

New MED added

No; the MED received on an EBGP link can be advertised via IBGP within the local AS

Yes

Local preference

Yes

No

Peering address

Normally loopback, recursive lookup provided by IGP, Time to Live (TTL) = 64

Normally peers directly to interface address, no recursion or IGP needed, TTL = 1

Update received from EBGP, is sent to

All IBGP peers

Other EBGP peers

Update received from IBGP, is sent to

No IBGP peers

All EBGP peers

Although the differences may seem trivial, they can have a significant impact. For example, because IBGP updates do not alter the AS path attribute, loops become a concern, and this leads to the restriction that IBGP speakers cannot readvertise an IBGP update to other IBGP speakers, which leads to the requirement that IBGP speakers should be fully meshed (see the next section for alternatives).

The next hop-handling differences often lead to IBGP routes that are hidden because the receiver cannot resolve the associated BGP next hop. By default, the next hop identifies the EBGP speaker in the adjoining AS, and often ...

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