BGP neighbors are routers forming TCP connection for exchanging BGP updates it’s also called as BGP peers or BGP speakers.
There are two types of BGP neighbor relationship
IBGP (internal BGP)
EBGP (external BGP)
BGP first forms a neighbor relationship with BGP speakers (peer). Than its learns information from its peer, BGP place that information in its BGP table and analyze the table to choose the best working route for each subnet in the BGP table, placing those routes into the IP routing table.
There are several requirements for forming BGP neighbors:-
A local router’s autonomous system number must match the neighboring router’s ASN.
The BGP router ID of the two routers must not be the same.
Each router must be part of a TCP connection.
Authentication must be passing if it’s configured
BGP message types
Open- it’s used to established a neighbor relationship and exchange its parameter, its include ASN and authentication values.
Keepaliveit’s used to maintain the neighbor relationship and it’s sent periodically
Update- its used to exchange path attribute.
Notification- it’s used error signals. It does occur in neighbor relationship reset.
BGP neighbor states
Idle– it’s a down state by administratively down or waiting for the next retry attempt.
R1# show ip bgp summary
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
18.104.22.168 4 501 11 9 8 0 0 00:04:28 4
22.214.171.124 4 501 0 0 1 0 0 never Idle
Connect- this state means its waiting for the TCP connection to be completed.
Active- this state means the TCP connection has been completed but there is no BGP messages sent yet to its peer.
Opensent- this state means the TCP connection has been established and BGP open message is sent to its BGP speakers but the matching Open message is not received yet from the peer router.
Openconfirm- this state means open message has been sent and received from BGP speaker router. But it’s waiting for keepalive message.
Established- this state means the entire neighbor parameters match and the neighbor relationship has been established. Now BGP speakers (peers) can exchange update messages.
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