JUNOS Enterprise Routing

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 5-1 summarizes the key differences.

Table 5-1. 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 must be fully meshed.

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 the IGP will not carry this route, thereby ...