La “capability” multisession consente al BGP di avere multiple sessioni BGP (quindi multiple conessioni TCP) tra una coppia di speekers, anche se esiste solo un stato di “neighbor”

La “mulitsession capability” è scambiata nei messaggi di “Open” all’atto dell’instaurazione sessione BGP come mostrato nell’output sottostante.

R1#debug ip bgp 10.100.1.2
BGP debugging is on for neighbor 10.100.1.2 for address family: IPv4 Unicast
  
We clear the BGP session.

BGP: 10.100.1.2 passive rcv OPEN, version 4, holdtime 180 seconds
BGP: 10.100.1.2 passive rcv OPEN w/ OPTION parameter len: 35
BGP: 10.100.1.2 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 4
BGP: 10.100.1.2 passive OPEN has CAPABILITY code: 132, length 2
BGP: 10.100.1.2 passive OPEN has TID CAP for:  2
BGP: 10.100.1.2 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 6
BGP: 10.100.1.2 passive OPEN has CAPABILITY code: 1, length 4
BGP: 10.100.1.2 passive OPEN has MP_EXT CAP for afi/safi: 1/1
BGP: 10.100.1.2 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 2
BGP: 40.50.1.1 passive OPEN has CAPABILITY code: 128, length 0
BGP: 40.50.1.1 passive OPEN has ROUTE-REFRESH capability(old) for all address-families
BGP: 40.50.1.1 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 2
BGP: 40.50.1.1 passive OPEN has CAPABILITY code: 2, length 0
BGP: 40.50.1.1 passive OPEN has ROUTE-REFRESH capability(new) for all address-families
BGP: 40.50.1.1 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 3
BGP: 40.50.1.1 passive OPEN has CAPABILITY code: 131, length 1
BGP: 40.50.1.1 passive OPEN has MULTISESSION capability, without grouping
BGP: 40.50.1.1 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 6
BGP: 40.50.1.1 passive OPEN has CAPABILITY code: 65, length 4
BGP: 40.50.1.1 passive OPEN has 4-byte ASN CAP for: 1
BGP: nbr global 40.50.1.1 neighbor does not have IPv4 MDT topology activated
BGP: 40.50.1.1 passive rcvd OPEN w/ remote AS 1, 4-byte remote AS 1

R1#show ip bgp neighbors 10.100.1.2
BGP neighbor is 10.100.1.2,  remote AS 1, internal link

  Neighbor capabilities:
    Route refresh: advertised and received(new) on session 1, 2, 3
    Four-octets ASN Capability: advertised and received on session 1, 2, 3
    Address family IPv4 Unicast: advertised and received
    Multisession Capability: advertised and received

Se vogliamo avere una sessione strettamente per neighobr possiamo utilizzare il comand “transport multi-session”

R1#sh run | s bgp
router bgp 1
neighbor 10.100.1.2 remote-as 1
neighbor 10.100.1.2 transport multi-session
neighbor 10.100.1.2 update-source Loopback0
!
address-family ipv4
  neighbor 10.100.1.2 activate
exit-address-family
!
address-family vpnv4
  neighbor 10.100.1.2 activate
  neighbor 10.100.1.2 send-community extended
exit-address-family
!
address-family ipv6
  neighbor 10.100.1.2 activate
exit-address-family

R2#

BGP: 10.100.1.1 passive open to 10.100.1.2

...

BGP: 10.100.1.1 passive OPEN has CAPABILITY code: 1, length 4
BGP: 10.100.1.1 passive OPEN has MP_EXT CAP for afi/safi: 1/1

...

BGP: 10.100.1.1 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 3
BGP: 10.100.1.1 passive OPEN has CAPABILITY code: 131, length 1
BGP: 10.100.1.1 passive OPEN has MULTISESSION capability, without grouping

BGP: ses global 10.100.1.1 (0x461CC60:0) pas Adding topology IPv4 Unicast:base

%BGP-5-ADJCHANGE: neighbor 10.100.1.1 Up

BGP: 10.100.1.1 passive open to 10.100.1.2

...

BGP: 10.100.1.1 passive rcvd OPEN w/ optional parameter type 2 (Capability) len 6
BGP: 10.100.1.1 passive OPEN has CAPABILITY code: 1, length 4
BGP: 10.100.1.1 passive OPEN has MP_EXT CAP for afi/safi: 2/1

...