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The BGP Accumulated IGP feature is required to simulate the current Open Shortest Path First (OSPF) behavior of computing the distance associated with a path. OSPF or Label Distribution Protocol (LDP) carries the prefix or label information only in the local area. Then, Border Gateway Protocol (BGP) carries the prefix or label to all the remote areas by redistributing the routes into BGP at area boundaries. The routes or labels are then advertised using label-switched paths (LSP). The next-hop for the route is changed at each Area Border Router (ABR) to a local device, which removes the need to leak OSPF routes across area boundaries. The bandwidth available on each of the core links is mapped to the OSPF cost; therefore, it is imperative that BGP carries this cost correctly between each of the provider edge (PE) devices. This functionality is achieved by using the BGP Accumulated IGP feature.

You need to enable accumulated interior gateway protocol (AIGP) processing for internal Border Gateway Protocol (iBGP) and external Border Gateway Protocol (eBGP) neighbors to carry the AIGP attribute. Neighbors configured with the AIGP attribute are put in a separate update group from other iBGP neighbors. A separate update group is required for neighbors that are enabled to send the AIGP value to cost community. BGP needs to translate the AIGP attribute to the cost community or multi-exit discriminator (MED) and attach it to the route before advertising to legacy.

When BGP installs AIGP attribute routes into the routing information base (RIB), it adds the AIGP cost with the next-hop cost. If the next-hop is a nonrecursive IGP route, BGP sets the AIGP metric to the received AIGP value and the first hop IGP metric to the next-hop. If the next-hop is a recursive route with the AIGP metric, BGP adds the received AIGP metric to the next-hop AIGP metric.

When a session receives a prefix with the accumulated interior gateway protocol (AIGP) attribute and is not configured to receive AIGP information, the session discards the AIGP attribute and processes the remainder of the update message, and then it passes the AIGP attribute to other BGP peers. The route is then installed into the routing information base (RIB) and the prefix is sent with the AIGP attribute to all the AIGP-enabled neighbors. The AIGP attribute value is not updated if the next-hop of the route is not changed by the device before advertising it to the neighbor. If the device changes the next-hop of the route, it recalculates the AIGP attribute value by adding the next-hop metric to the received AIGP attribute value.

Origination of routes with the accumulated interior gateway protocol (AIGP) metric is controlled by configuration. AIGP attributes are attached to redistributed routes that satisfy the following conditions:

• The protocol redistributing the route is enabled for AIGP.

• The route is an interior gateway protocol (IGP) route redistributed into Border Gateway Protocol (BGP). The value assigned to the AIGP attribute is the value of the IGP next-hop to the route or as set by a route policy.

• The route is a static route redistributed into BGP. The value assigned is the value of the next-hop to the route or as set by a route policy.

• The route is imported into BGP through a network statement. The value assigned is the value of the next-hop to the route or as set by a route policy.

• The inbound or outbound route map also creates an AIGP attribute route map using the set aigp-metric command.

1.    enable


2.    configure terminal


3.    router bgp as-number


4.    address-family ipv4 [unicast | multicast | vrf vrf-name]


5.    redistribute protocol autonomous-system-number route-map map-tag


6.    network network-id route-map map-tag


7.    exit


8.    route-map rtmap


9.    set aigp-metric [igp-metric | value]


10.    end

Device> enable

Device# configure terminal

Device(config)# router bgp 40000

Device(config-router)# address-family ipv4 unicast

Device(config-router-af)# redistribute bgp 100 route-map rtmap 

Device(config-router-af)# network 10.1.1.1 route-map rtmap 

Device(config-router-af)# exit

Device(config)# route-map rtmap

Device(config-route-map)# set aigp-metric igp-metric

Device(config-route-map)# end

1.    enable


2.    configure terminal


3.    router bgp as-number


4.    address-family {ipv4 | ipv6} [unicast]


5.    neighbor ip-address aigp


6.    end

Device> enable

Device# configure terminal

Device(config)# router bgp 40000

Device(config-router)# address-family ipv4 unicast

Device(config-router-af)# neighbor 192.168.1.1 aigp

Device(config-router-af)# end

1.    enable


2.    configure terminal


3.    router bgp as-number


4.    address-family {ipv4 | ipv6} [unicast]


5.    neighbor ip-address aigp [send {cost-community community-id poi {igp-cost | pre-bestpath} [transitive]} | med]


6.    end

Device> enable

Device# configure terminal

Device(config)# router bgp 40000

Device(config-router)# address-family ipv4 unicast

Device(config-router-af)# neighbor 192.168.1.1 aigp send med

Device(config-router-af)# end