Connectivity Guide

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.
OpenConfirm 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 notification messages to and from its
peer.
Peer templates
Peer templates allow BGP neighbors to inherit the same outbound policies. Instead of manually configuring 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
efficient update calculation with a simplified configuration.
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 reflectors
Route reflectors (RRs) reorganize the IBGP core into a hierarchy and allow route advertisement rules. Route reflection divides
IBGP peers into two groups — client peers and nonclient peers.
● If a route is received from a nonclient peer, it reflects the route to all client peers
● If a route is received from a client peer, it reflects the route to all nonclient and client peers
An RR and its client peers form a route reflection cluster. BGP speakers announce only the best route for a given prefix. 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 reflection cluster,
where Router D is the route reflector. 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.
2. Router C receives the advertisement but does not advertise it to any peer because its only other peer is Router D (an IBGP
peer) and Router D has already learned it through IBGP from Router B.
3. Router D does not advertise the route to Router C because Router C is a nonclient peer. The route advertisement came from
Router B which is also a nonclient peer.
4. Router D does reflect the advertisement to Routers E and G because they are client peers of Router D.
5. Routers E and G advertise this IBGP learned route to their EBGP peers — Routers F and H.
Layer 3
411