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Conditional BGP Route Origination

Sebastian Majewski has found an interesting feature: if you use the network route-map BGP configuration command to originate BGP prefixes and use the match conditions within the route-map, BGP inserts the IP prefix in the BGP table only if the source route in the IP routing table satisfies the route-map conditions.

The following text written by Ivan Pepelnjak in 2008 was originally published on CT3 wiki. That web site became unreachable in early 2019. We retrieved the original text from the Internet Archive, cleaned it up, updated it with recent information if necessary, and republished it on ipSpace.net blog on December 28, 2020

More Details

The network a.b.c.d mask e.f.g.h route-map name BGP router configuration command is used to set attributes of the BGP prefix inserted in the BGP table when the corresponding IP prefix is installed in the IP routing table. Common usages include setting BGP communities, BGP local-preference or Multi-Exit Discriminator (MED).

Unless configured in a route-map, MED is copied from the metric of the original route, local-preference is set to the value of the bgp default local-preference (or left empty) and BGP communities are not set.

The route-map used in the network command can also match some attributes of the route in the IP routing table (but not in original routing protocol database). In this case, the BGP prefix will be inserted only if the original route matches the requirements of the route-map.

Example

In a network using floating static routes for backup purposes, you might want to advertise the IP prefixes into BGP only if the subnets are reachable via a routing protocol, not via a floating static route.

The match source-protocol configuration command within a route-map does not work when used with the network command, so you have to use route tagging:

  • Floating static route is marked with a tag;
  • Routes received through routing protocols don’t have tags;
  • match tag condition is used within a route-map deny statement to prevent the static route from being inserted into the BGP table.

The relevant parts of the router configuration that conditionally originates the IP prefix 10.3.1.0/24 are displayed in the following printout:

router bgp 65000
 network 10.3.1.0 mask 255.255.255.0 route-map NoStatic
!
ip route 10.3.1.0 255.255.255.0 Serial1/0 250 tag 250
!
router ospf 1
 network 0.0.0.0 255.255.255.255 area 0
!
route-map NoStatic deny 10
 match tag 250
!
route-map NoStatic permit 20
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