Friday, March 13, 2009

Simple Setup for Dual Link ISP

I know a couple of customers that do run BGP just to ensure failover for there internet connectivity. The headache of running BGP with ISP can be saved by using few tricks, i shouldn't say it trick, rather its a feature of Cisco IOS.

A draw back of ehternet interface is that its status remain same regardless of far side interface status, this cause serious issues as for routing process as link remains up even the remote router ethernet interface fails to respond and thus the invalid route remain in routing table.

IP SLA can send echo packet to remote router and upon failure to receive ICMP reply can trigger the any specifed event like installing less prefered route to routing table and remove the old one below is the sample config of above topology


ip sla monitor 1 // Enter a number to reference our monitor

type echo protocol ipIcmpEcho // is the ISP A IP i.e. Far side IP address of remote router)
timeout 500
frequency 3
ip sla monitor schedule 1 life forever start-time now // Scedule SLA to run forever
track 1 rtr 1 reachability // Creates tracked object 1 for RTR (Response time Reporter)
interface FastEthernet0/0
// Connection to ISP A

ip address
duplex auto
speed auto
interface FastEthernet1/0

// Connection to ISP B
ip address
duplex auto
speed auto
Now Time for routing (:
ip route track 1
(Add primary route, installed if track 1 is successfully completed)
ip route 20 (Secondary route to ISP B, should activate if Link to ISP A fails) =====================================================================
State 1: ISP A is active

R0#sh ip route {Output ommited}
S* [1/0] via

R0#sh ip route track-table
ip route track 1 state is [up] ======================================================================
State 2:
ISP A is inactive( I had shutdown R1 F0/0 to simulate it)
R0#sh ip route track-table
ip route track 1 state is [down]
R0#sh ip route
S* [20/0] via (Notice routing table has installed new default route to ISP B) ======================================================================

Mine one is just starter for more details and digging it with NAT etc below are very good links

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