how do i configure an alternate default route through my BGP network

Answered Question
Apr 18th, 2010

I have a telstra provided IP WAN network running  BGP.

I advertise 0.0.0.0 from my main site by the network 0.0.0.0 command under router bgp 61111, at the Disaster recovery(DR) site, I  have the network 0.0.0.0 backdoor under BGP 62222 because there is  internal eigrp running over a gigabit link between the two sites. This all works ok. Now if the main Router IP WAN link goes down, is there anyway that the DR can start advertising the default route automatically

I have this problem too.
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Correct Answer by Giuseppe Larosa about 6 years 7 months ago

Hello Chrbardf1,

How are the sites interconnected to main site and DR site?

I will suppose that you have an MPLS L3 VPN service where each site peers with SP PE node using eBGP sesssions.

IF this is the case it should be enough for the DR site CE node that you manage to advertise a default route with appropriate AS path prepending

example:

ip prefix-list only-default permit 0.0.0.0/0

route-map selective_prepend permit 10

match ip address prefix only-default

set as-path prepend 62222 62222

route-map selective_prepend permit 20

router bgp 62222

neigh ISP-peer remote-as ISP-ASN

neigh ISP-peer route-map selective_prepend out

!

This should provide a default route that is used only when primary route at HQ disappears

if your network scenario is different the above solution does not apply to it.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sun, 04/18/2010 - 23:45

Hello Chrbardf1,

How are the sites interconnected to main site and DR site?

I will suppose that you have an MPLS L3 VPN service where each site peers with SP PE node using eBGP sesssions.

IF this is the case it should be enough for the DR site CE node that you manage to advertise a default route with appropriate AS path prepending

example:

ip prefix-list only-default permit 0.0.0.0/0

route-map selective_prepend permit 10

match ip address prefix only-default

set as-path prepend 62222 62222

route-map selective_prepend permit 20

router bgp 62222

neigh ISP-peer remote-as ISP-ASN

neigh ISP-peer route-map selective_prepend out

!

This should provide a default route that is used only when primary route at HQ disappears

if your network scenario is different the above solution does not apply to it.

Hope to help

Giuseppe

Richard Bradfield Mon, 04/19/2010 - 16:31

I thought the prepend commands fixed the problem,

but i want to advertise 172.16.17.0/24, and 172.17.6.0/24 out of the DR site, but adding the commands as above stopped those networks being advertised accross the BGP network

I tried using  the prefix-list as below but that did not make any difference

ip prefix-list test deny 172.16.17.0/24

ip prefix-list test deny 172.17.6.0/24

ip prefix-list test permit 0.0.0.0/0

Richard

Giuseppe Larosa Mon, 04/19/2010 - 22:43

Hello Richard,

>> route-map selective_prepend permit 20

an empty final route-map clause (like the one above) is needed to keep advertise IP prefixes that you don't want to prepend but you need to advertise

Hope to help

Giuseppe

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