BGP routing help

[kartik.shah1]

Hi,

I have two 1941 routers in network. I have two different ISPs. Each router is running BGP with ISP router. (scenario is attached below).

As shown in scenario, i want ISP A as primary. My all traffic should route from ISP A. But whenever ISP A fails, traffic should route from ISP B.

I am planning to configure HSRP. But what BGP configurations, i have to do on Cisco 1941 routers.

thank you,

regards,,

[milan.kulik]

Hi,

for incoming traffic to prefer your router A, I'd say the easiest way would be to advertise your own prefixes to ISP B with your AS number 100 prepended several times.

That way most Internet devices should prefer the path to your network through ISP A.

For outgoing traffic:

1) The simplest way would be to configure HSRP on your LAN side to prefer router A. You would need to track BGP connectivity to ISP A then and decrement HSRP priority to failover to router B in a case the BGP connectivity to ISP A fails.

2) Another possibility would be configuring iBGP between your routers and preferring (increasing Local Preference, e.g.) prefixes received from ISP A.

Check carefully you are advertising only your own prefixes in this case (not to become a transit site for your ISPs.)!

Best regards,

Milan

[kartik.shah1]

Hi milan.kulik,

thank you for replying.

Can you please list down configuration commands for same.

regards,,

[Richard Bradfield]

Hi,

On your diagram you have AS100, so does that mean you have BGP AS100 from both ISPs?

anyway l would do the following

I would have the switch and routers running Eigrp

the highlighted metric is the bandwidth, so normally the route to the internet is advertised to the switch thru router A as its bandwidth is 2000 whereas Router B is 1000. if the route thru BGP is lost then automatically the route thru Router B is advertised to the switch.

then on Router A

router eigrp 1

default-metric 2000 100 255 1 1500

network <the subnet of your link to the switch>

redistribute bgp 100

Router B

router eigrp 1

default-metric 1000 100 255 1 1500

network <the subnet of your link to the switch>

redistribute bgp 100

Switch

router eigrp 1

network <one entry for each attached network>

HTH

Richard

[milan.kulik]

Hi,

IMHO, the safest way would be to configure HSRP to track some route (prefix) advertised by your ISP A (it would be nice to agree with ISP A to advertise some loopback to you, e.g.).

Look at

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_52_se/configuration/guide/3560scg/sweot.html#wp1084432

Best regards,

Milan