Edge-Core ECS4660-28F, Management Manual, Page: 1819 / 2020

C
HAPTER
51
| IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1819 –
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
Router
Router
Router
AS200
Router
Router
Router
Router
AS100
Router
Router
Router
Router
AS300
eBGP
eBGP eBGP
eBGP
iBGP
iBGP iBGP