There is a formula that you can calculate how many sessions is needed in network. Let’s say that you have 10 routers and that you have configure peer sessions with all routers in AS.
N*(N-1)/2
N represent total number of routers in AS. For example let’s say that we have 10 routers, we come up to 45 sessions that must be configured.
There is tool that help as to reduce number of needed sessions in network and that is route reflector.
Route reflection allows creating peer relationships only with designated route reflector router.
In that case you will need only IBGP peer session between route reflector clients and selected route reflector router.
Router reflector and router reflector clients must be in the same cluster.
You can have more than one router reflector router under same AS, also to enable connectivity for router reflector client and nonclient routers under AS, route reflector router must be full meshed with all routers in AS.
To achieve full mash you will need to configure multiple neighbor statements on each IBGP device.
For that reason in large networks full mash does not scale well.
The best practice is to enable "route reflector", select one router on your network to be reflector router (Router_1), create peer relationships with all other routers under AS from route reflector router.
Then select routers that you want them to be clients for that router reflector (Router_2 and Router_3) all other routers are nonclient’s routers (Router_4 and Router_5).
Under Router reflector (Router_1) use cluster command and cluster ID (any 32 bit value); best practice is to use router loopback interface address.
Router reflector clients (Router_2 and Router_3) must form a peer relationship only with RF (Router_1) and RF clients don’t have to peer between them.
Nonclient routers (Router_4 and Router_5) must peer with each other and route reflection router (Router_1), use neighbor statement on each nonclient device (Router_4 to Router_5) and (Router_5 to Router_4), nonclient routers (Router_4 and Router_5) does not need to form peer relationship with router reflector clients (Router_2 and Router_3).
Router_1
set routing-options router-id 192.168.6.5
set routing-options autonomous-system 200
set protocols bgp group INTERNAL-PEERS type internal
set protocols bgp group INTERNAL-PEERS local-address 192.168.6.5
set protocols bgp group INTERNAL-PEERS export SEND-OSPF
set protocols bgp group INTERNAL-PEERS cluster 192.168.6.5
set protocols bgp group INTERNAL-PEERS neighbor 192.168.6.4
set protocols bgp group INTERNAL-PEERS neighbor 192.168.40.4
set protocols bgp group INTERNAL-PEERS neighbor 192.168.5.5
set protocols bgp group INTERNAL-PEERS neighbor 192.168.0.1
set protocols ospf area 0.0.0.0 interface all
set protocols ospf area 0.0.0.0 interface lo0.1 passive
set policy-options policy-statement SEND-OSPF term 2 from protocol ospf
set policy-options policy-statement SEND-OSPF term 2 then accept
Router_2
set routing-options router-id 192.168.6.4
set routing-options autonomous-system 200
set protocols bgp group INTERNAL-PEERS type internal
set protocols bgp group INTERNAL-PEERS local-address 192.168.6.4
set protocols bgp group INTERNAL-PEERS export SEND-OSPF
set protocols bgp group INTERNAL-PEERS neighbor 192.168.6.5
set protocols ospf area 0.0.0.0 interface all
set protocols ospf area 0.0.0.0 interface lo0.2 passive
set policy-options policy-statement SEND-OSPF term 2 from protocol ospf
set policy-options policy-statement SEND-OSPF term 2 then accept
Router_3
set routing-options router-id 19.168.40.4
set routing-options autonomous-system 200
set protocols bgp group INTERNAL-PEERS type internal
set protocols bgp group INTERNAL-PEERS local-address 192.168.40.4
set protocols bgp group INTERNAL-PEERS export SENT-OSPF
set protocols bgp group INTERNAL-PEERS neighbor 192.168.6.5
set protocols ospf area 0.0.0.0 interface all
set protocols ospf area 0.0.0.0 interface lo0.0 passive
set policy-options policy-statement SENT-OSPF term 2 from protocol ospf
set policy-options policy-statement SENT-OSPF term 2 then accept
Router_4
set routing-options router-id 192.168.0.1
set routing-options autonomous-system 200
set protocols bgp group INTERNAL-PEERS type internal
set protocols bgp group INTERNAL-PEERS local-address 192.168.0.1
set protocols bgp group INTERNAL-PEERS export SEND-OSPF
set protocols bgp group INTERNAL-PEERS neighbor 192.168.6.5
set protocols bgp group INTERNAL-PEERS neighbor 192.168.5.5
set protocols ospf area 0.0.0.0 interface all
set protocols ospf area 0.0.0.0 interface lo0.4 passive
set policy-options policy-statement SEND-OSPF term 2 from protocol ospf
set policy-options policy-statement SEND-OSPF term 2 then accept
Router_5
set routing-options router-id 192.168.5.5
set routing-options autonomous-system 200
set protocols bgp group INTERNEL-PEERS type internal
set protocols bgp group INTERNEL-PEERS local-address 192.168.5.5
set protocols bgp group INTERNEL-PEERS export SEND-OSPF
set protocols bgp group INTERNEL-PEERS neighbor 192.168.0.1
set protocols bgp group INTERNEL-PEERS neighbor 192.168.6.5
set protocols ospf area 0.0.0.0 interface all
set protocols ospf area 0.0.0.0 interface lo0.5 passive
set policy-options policy-statement SEND-OSPF term 2 from protocol ospf
set policy-options policy-statement SEND-OSPF term 2 then accept
Verifying BGP Neighbors
root@Router_1> show bgp summary
Groups: 1 Peers: 4 Down peers: 0
Table Tot Paths Act Paths Suppressed History Damp State Pending
inet.0 26 0 0 0 0 0
Peer AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
192.168.0.1 200 208 202 0 0 1:27:17 0/6/1/0 0/0/0/0
192.168.5.5 200 187 182 0 0 1:20:00 0/7/7/0 0/0/0/0
192.168.6.4 200 172 157 0 0 1:08:53 0/6/1/0 0/0/0/0
192.168.40.4 200 168 156 0 0 1:08:53 0/7/7/0 0/0/0/0
root@Router_1> show bgp neighbor
Peer: 192.168.0.1+50052 AS 200 Local: 192.168.6.5+179 AS 200
Type: Internal State: Established (route reflector client)Flags: <ImportEval Sync>
Last State: OpenConfirm Last Event: RecvKeepAlive
Last Error: None
Export: [ SEND-OSPF ]
Options: <Preference LocalAddress Cluster Refresh>
Local Address: 192.168.6.5 Holdtime: 90 Preference: 170
Number of flaps: 0
Peer ID: 192.168.0.1 Local ID: 192.168.6.5 Active Holdtime: 90
Keepalive Interval: 30 Peer index: 0
BFD: disabled, down
NLRI for restart configured on peer: inet-unicast
NLRI advertised by peer: inet-unicast
NLRI for this session: inet-unicast
Peer supports Refresh capability (2)
Stale routes from peer are kept for: 300
Peer does not support Restarter functionality
NLRI that restart is negotiated for: inet-unicast
NLRI of received end-of-rib markers: inet-unicast
NLRI of all end-of-rib markers sent: inet-unicast
Peer supports 4 byte AS extension (peer-as 200)
Peer does not support Addpath
Table inet.0 Bit: 10000
RIB State: BGP restart is complete
Send state: in sync
Active prefixes: 0
Received prefixes: 6
Accepted prefixes: 1
Suppressed due to damping: 0
Advertised prefixes: 6
Last traffic (seconds): Received 4 Sent 6 Checked 68
Input messages: Total 212 Updates 9 Refreshes 0 Octets 4340
Output messages: Total 205 Updates 7 Refreshes 0 Octets 4208
Output Queue[0]: 0
Peer: 192.168.5.5+51392 AS 200 Local: 192.168.6.5+179 AS 200
Type: Internal State: Established (route reflector client)Flags: <ImportEval Sync>
Last State: OpenConfirm Last Event: RecvKeepAlive
Last Error: None
Export: [ SEND-OSPF ]
Options: <Preference LocalAddress Cluster Refresh>
Local Address: 192.168.6.5 Holdtime: 90 Preference: 170
Number of flaps: 0
Peer ID: 192.168.5.5 Local ID: 192.168.6.5 Active Holdtime: 90
Keepalive Interval: 30 Peer index: 1
BFD: disabled, down
NLRI for restart configured on peer: inet-unicast
NLRI advertised by peer: inet-unicast
NLRI for this session: inet-unicast
Peer supports Refresh capability (2)
Stale routes from peer are kept for: 300
Peer does not support Restarter functionality
NLRI that restart is negotiated for: inet-unicast
NLRI of received end-of-rib markers: inet-unicast
NLRI of all end-of-rib markers sent: inet-unicast
Peer supports 4 byte AS extension (peer-as 200)
Peer does not support Addpath
Table inet.0 Bit: 10000
RIB State: BGP restart is complete
Send state: in sync
Active prefixes: 0
Received prefixes: 7
Accepted prefixes: 7
Suppressed due to damping: 0
Advertised prefixes: 6
Last traffic (seconds): Received 9 Sent 21 Checked 77
Input messages: Total 190 Updates 9 Refreshes 0 Octets 3927
Output messages: Total 185 Updates 5 Refreshes 0 Octets 3768
Output Queue[0]: 0
Peer: 192.168.6.4+179 AS 200 Local: 192.168.6.5+57627 AS 200
Type: Internal State: Established (route reflector client)Flags: <ImportEval Sync>
Last State: OpenConfirm Last Event: RecvKeepAlive
Last Error: None
Export: [ SEND-OSPF ]
Options: <Preference LocalAddress Cluster Refresh>
Local Address: 192.168.6.5 Holdtime: 90 Preference: 170
Number of flaps: 0
Peer ID: 192.168.6.4 Local ID: 192.168.6.5 Active Holdtime: 90
Keepalive Interval: 30 Peer index: 2
BFD: disabled, down
NLRI for restart configured on peer: inet-unicast
NLRI advertised by peer: inet-unicast
NLRI for this session: inet-unicast
Peer supports Refresh capability (2)
Stale routes from peer are kept for: 300
Peer does not support Restarter functionality
NLRI that restart is negotiated for: inet-unicast
NLRI of received end-of-rib markers: inet-unicast
NLRI of all end-of-rib markers sent: inet-unicast
Peer supports 4 byte AS extension (peer-as 200)
Peer does not support Addpath
Table inet.0 Bit: 10000
RIB State: BGP restart is complete
Send state: in sync
Active prefixes: 0
Received prefixes: 6
Accepted prefixes: 1
Suppressed due to damping: 0
Advertised prefixes: 6
Last traffic (seconds): Received 7 Sent 1 Checked 15
Input messages: Total 176 Updates 5 Refreshes 0 Octets 3506
Output messages: Total 160 Updates 4 Refreshes 0 Octets 3261
Output Queue[0]: 0
Peer: 192.168.40.4+58282 AS 200 Local: 192.168.6.5+179 AS 200
Type: Internal State: Established (route reflector client)Flags: <ImportEval Sync>
Last State: OpenConfirm Last Event: RecvKeepAlive
Last Error: None
Export: [ SEND-OSPF ]
Options: <Preference LocalAddress Cluster Refresh>
Local Address: 192.168.6.5 Holdtime: 90 Preference: 170
Number of flaps: 0
Peer ID: 19.168.40.4 Local ID: 192.168.6.5 Active Holdtime: 90
Keepalive Interval: 30 Peer index: 3
BFD: disabled, down
NLRI for restart configured on peer: inet-unicast
NLRI advertised by peer: inet-unicast
NLRI for this session: inet-unicast
Peer supports Refresh capability (2)
Stale routes from peer are kept for: 300
Peer does not support Restarter functionality
NLRI that restart is negotiated for: inet-unicast
NLRI of received end-of-rib markers: inet-unicast
NLRI of all end-of-rib markers sent: inet-unicast
Peer supports 4 byte AS extension (peer-as 200)
Peer does not support Addpath
Table inet.0 Bit: 10000
RIB State: BGP restart is complete
Send state: in sync
Active prefixes: 0
Received prefixes: 7
Accepted prefixes: 7
Suppressed due to damping: 0
Advertised prefixes: 6
Last traffic (seconds): Received 3 Sent 22 Checked 66
Input messages: Total 172 Updates 5 Refreshes 0 Octets 3475
Output messages: Total 159 Updates 4 Refreshes 0 Octets 3242
Output Queue[0]: 0
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