BGP - default route origination from secondary site

Unanswered Question
Feb 11th, 2010

I would like someone smarter than me to look at this config and let me know if it will work. In the example, Memphis is the home office, and when up and working, the Memphis WAN router originates the default route to the MPLS network:

int s2/0.100 point-to-point

ip addr 152.177.98.78 255.255.255.252


router bgp 845

neighbor 152.177.93.77 remote-as 65000

neighbor 152.177.93.77 default-originate

The secondary site, New York, must be able to accept the default origination when Memphis is up, but then NY must originate the default route if/when Memphis goes down. I have that configured as follows:

int s2/0.100 point-to-point

ip addr 152.177.156.90


router bgp 845

neighbor 152.177.156.89 remote-as 65000

neighbor 152.177.156.89 default-originate route-map DEFROUTE


route-map DEFROUTE deny 10

match ip address 1


access-list 1 permit 152.177.93.78

I have also considered that the route-map and access-list could be reversed and still work, but I'm not sure:

route-map DEFROUTE permit 10

match ip address 1


access-list 1 deny 152.177.93.78

Any help would be greatly appreciated. Thanks.

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Giuseppe Larosa Thu, 02/11/2010 - 22:43

Hello Billy,

what you have done up to now is interesting but it doesn't work. You are trying to match a prefix but in the ACL you then try to match a BGP next-hop.

to match a BGP next-hop in a route map we should use a different match statement:

match ip next-hop acl#

but your NY router sees a different BGP next-hop the one provided by PE router (if this is an MPLS L3 VPN service).

There is an interesting feature for cases like yours called BGP conditional advertising.

see

http://www.cisco.com/en/US/docs/ios/12_1/iproute/configuration/guide/1cdbgp.html#wp1023342

neighbor ip-address advertise-map map1 non-exist-map map2

this implements the logic that you would like to use. Route-maps are used to provide what to advertise when some prefix disappears.

I'm not sure it can be used if you only receive the default route on the eBGP session.

see also

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml

Another possibility would be to provide to default route received from Memphis an higher weight then locally generated prefixes for example using

neighbor 152.177.156.89 weight 40000

the NY router would choice as best a default route received on the session and it will not advertise its own until the other one is alive.

This should be the simplest way to implement this.

neigh 152.177.156.89 weight 40000

neigh 152.177.156.89 default-originate

Hope to help

Giuseppe

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