Wednesday, 4 May 2011

EBGP Multihop

Multihop EBGP sessions are the traditional way to implement EBGP load balancing on parallel links. EBGP session is established between loopback interfaces of adjacent routers (see the diagram above; router configurations are included at the bottom of the article) and static routes (or an extra instance of a dynamic routing protocol) are used to achieve connectivity between loopback interfaces (BGP next-hops). The load balancing is an automatic result of the recursive route lookup of BGP next hops.
EBGP session between loopback interfaces is most appropriate in scenarios where all the links between the EBGP neighbors have identical bandwidth. To achieve proportional load balancing across links with different bandwidths, you could use EIGRP as the dynamic routing protocol or single-hop MPLS traffic engineering. However, it’s better to use parallel EBGP sessions in the unequal-bandwidth scenarios.
“An EBGP multihop session (neighbor ebgp-multihop) is vulnerable. You can use the neighbor disable-connected-check command to have a single-hop EBGP session with an IP address that is not directly connected.”
Configuration:
R1
----------------------------------------------------------------
interface Loopback0
ip address 10.0.0.1 255.255.255.0
!
interface FastEthernet0/0
ip address 10.0.2.1 255.255.255.0
!
interface Serial2/0
ip address 10.0.1.2 255.255.255.252
serial restart-delay 0
clock rate 9600
!
interface Serial2/1
ip address 10.0.1.10 255.255.255.252
serial restart-delay 0
clock rate 9600
!
router bgp 64800
no synchronization
bgp log-neighbor-changes
neighbor 10.0.0.23 remote-as 65000
neighbor 10.0.0.23 disable-connected-check
neighbor 10.0.0.23 update-source Loopback0
no auto-summary
!
Ip route 10.0.0.23 255.255.255.255 10.0.1.1
Ip route 10.0.0.23 255.255.255.255 10.0.1.9
!

R2
--------------------------------------------------
interface Loopback0
ip address 10.0.0.23 255.255.255.0
!
interface FastEthernet0/0
ip address 10.7.1.1 255.255.255.0
!
interface Serial2/0
ip address 10.0.1.1 255.255.255.252
serial restart-delay 0
!
interface Serial2/1
ip address 10.0.1.9 255.255.255.252
serial restart-delay 0

router bgp 65000
no synchronization
bgp log-neighbor-changes
neighbor 10.0.0.1 remote-as 64800
neighbor 10.0.0.1 disable-connected-check
neighbor 10.0.0.1 update-source Loopback0
no auto-summary
!
Ip route 10.0.0.1 255.255.255.255 10.0.1.2
Ip route 10.0.0.1 255.255.255.255 10.0.1.10
!
Verification:
R1# show ip bgp summary
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.23 4 65000 30 30 4 0 0 00:26:24 0
 

No comments:

Post a Comment