Connectivity Guide

Idle BGP initializes all resources, refuses all inbound BGP connection attempts, and starts a TCP connection to the
peer.
Connect Router waits for the TCP connection to complete and transitions to the OpenSent state if successful. If that
transition is not successful, BGP resets the ConnectRetry timer and transitions to the Active state when the
timer expires.
Active Router resets the ConnectRetry timer to zero and returns to the Connect state.
OpenSent Router sends an Open message and waits for one in return after a successful OpenSent transition.
OpenConrm Neighbor relation establishes and is in the OpenConfirm state after the Open message parameters are agreed on
between peers. The router then receives and checks for agreement on the parameters of the open messages to
establish a session.
Established Keepalive messages exchange, and after a successful receipt, the router is in the Established state. Keepalive
messages continue to send at regular periods. The keepalive timer establishes the state to verify connections.
After the connection is established, the router sends and receives keepalive, update, and notication messages to and from its peer.
Peer templates
Peer templates allow BGP neighbors to inherit the same outbound policies. Instead of manually conguring each neighbor with the same
policy, you can create a peer group with a shared policy that applies to individual peers. A peer template provides ecient update
calculation with a simplied conguration.
Peer templates also aid in convergence speed. When a BGP process sends the same information to many peers, a long output queue may
be set up to distribute the information. For peers that are members of a peer template, the information is sent to one place then passed on
to the peers within the template.
Route reectors
Route reectors (RRs) reorganize the IBGP core into a hierarchy and allow route advertisement rules. Route reection divides IBGP peers
into two groups — client peers and nonclient peers.
• If a route is received from a nonclient peer, it reects the route to all client peers
• If a route is received from a client peer, it reects the route to all nonclient and client peers
An RR and its client peers form a route reection cluster. BGP speakers announce only the best route for a given prex. RR rules apply
after the router makes its best path decision.
NOTE
: Do not use RRs in forwarding paths — hierarchal RRs that maintain forwarding plane RRs could create route loops.
Routers B, C, D, E, and G are members of the same AS—AS100. These routers are also in the same route reection cluster, where Router D
is the route reector. Routers E and G are client peers of Router D, and Routers B and C and nonclient peers of Router D.
1 Router B receives an advertisement from Router A through EBGP. Because the route is learned through EBGP, Router B advertises it
to all its IBGP peers — Routers C and D.
Layer 3
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