Next-hop and fully-meshed configuration

Posted in NETWORKING/IP ROUTING by edeguzman on October 3, 2007

VailV4#sh run
Building configuration…

Current configuration:
!
version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname VailV4
!
enable secret 5 $1$OiVL$E8vhRllZnn3MFwXkecM2I1
!
!
!
!
!
ip subnet-zero
no ip domain-lookup
ip host tahoe 192.168.1.210
ip host taos 192.168.1.225
ip host aspen 192.168.1.222
!
lane client flush
!
!
!
!
!
!
!
interface Ethernet0/0
ip address 192.168.1.221 255.255.255.252
!
interface Ethernet0/1
no ip address
shutdown
!
interface Serial1/0
ip address 192.168.1.226 255.255.255.252
no fair-queue
clockrate 64000
!
interface Serial1/1
ip address 192.168.1.209 255.255.255.252
clockrate 64000
!
interface Serial1/2
no ip address
shutdown
!
interface Serial1/3
no ip address
shutdown
!
router bgp 100
no synchronization
network 192.168.1.208 mask 255.255.255.252
network 192.168.1.224 mask 255.255.255.252
neighbor 192.168.1.197 remote-as 100
neighbor 192.168.1.197 next-hop-self
neighbor 192.168.1.210 remote-as 300
neighbor 192.168.1.222 remote-as 100
neighbor 192.168.1.222 next-hop-self
neighbor 192.168.1.225 remote-as 200
!
ip classless
no ip http server
!
!
!
line con 0
transport input none
line aux 0
line vty 0 4
password passkey
login
!
end

VailV4#

This configuration shows that the router vail was set up as a next-hop self. Wherein it simply means that the next_hop attribute for every update that it will send to the routers within the AS.

The configuration also shows that it is configured in fully meshed because the router
is configured as neighbor in every router in the AS.

As you also observed that there is a no synchronization configured in the BGP routing process. This is one tough stuff that I find it quite confusing until I grasp the concept on it. In CISCO routers the by default they are configured as synchronization, in that sense before a route learned from an IBGP neighbor is entered into the IGP routing table or before it is being advertised to a BGP peer, the route must be first be known via its IGP. The running IGP in the AS will track every posible route to the AS, in that sense a router with in the AS must know the route going to the destination before the
route will be place in the routing table.. Now when there is no synchronization, the routes in the BGP table will be place to the routing table even if the path going to the destination is unknown, that’s why fully meshed must be configured and all networks and subnets connecting to the IBGP routers must be known.

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