BGP Accumulated IGP
The BGP Accumulated IGP feature is an optional nontransitive Border Gateway Protocol (BGP) path attribute. The attribute type code for the accumulated interior gateway protocol (AIGP) attribute is assigned by the Internet Assigned Numbers Authority (IANA). The value field of the AIGP attribute is defined as a set of type, length, value (TLV) elements. The AIGP TLV contains the AIGP metric.
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Information About BGP Accumulated IGP
Overview of BGP Accumulated IGP
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.
Sending and Receiving BGP Accumulated IGP
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.
Originating Prefixes with Accumulated IGP
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.
How to Configure BGP Accumulated IGP
Configuring AIGP Metric Value
SUMMARY STEPS1.
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
DETAILED STEPS| | Command or Action | Purpose |
|---|
| Step 1 |
enable
Example:
|
Enables privileged EXEC mode.
|
| Step 2 |
configure
terminal
Example:
Device# configure terminal
|
Enters global configuration mode.
|
| Step 3 |
router
bgp
as-number
Example:
Device(config)# router bgp 40000
|
Enters router configuration mode for the specified routing process.
|
| Step 4 |
address-family
ipv4
[unicast |
multicast |
vrf
vrf-name]
Example:
Device(config-router)# address-family ipv4 unicast
|
Specifies the IPv4 address family and enters address family configuration mode.
|
| Step 5 |
redistribute
protocol
autonomous-system-number
route-map
map-tag
Example:
Device(config-router-af)# redistribute bgp 100 route-map rtmap
|
Redistributes routes from one routing domain to another routing domain.
|
| Step 6 |
network
network-id
route-map
map-tag
Example:
Device(config-router-af)# network 10.1.1.1 route-map rtmap
|
Specifies the networks to be advertised by the Border Gateway Protocol (BGP) routing process.
|
| Step 7 |
exit
Example:
Device(config-router-af)# exit
|
Exits address family configuration mode and returns to global configuration mode.
|
| Step 8 |
route-map rtmap
Example:
Device(config)# route-map rtmap
|
Enters route map configuration mode.
|
| Step 9 |
set aigp-metric
[igp-metric | value]
Example:
Device(config-route-map)# set aigp-metric igp-metric
|
Specifies a metric value for the accumulated interior gateway protocol (AIGP) attribute. The manual metric value range is from 0 to 4294967295.
|
| Step 10 |
end
Example:
Device(config-route-map)# end
|
Exits route map configuration mode and enters privileged EXEC mode.
|
Enabling Send and Receive for an AIGP Attribute
SUMMARY STEPS1.
enable
2.
configure
terminal
3.
router
bgp
as-number
4.
address-family
{ipv4 | ipv6}
[unicast]
5.
neighbor
ip-address
aigp
6.
end
DETAILED STEPS| | Command or Action | Purpose |
|---|
| Step 1 |
enable
Example:
|
Enables privileged EXEC mode.
|
| Step 2 |
configure
terminal
Example:
Device# configure terminal
|
Enters global configuration mode.
|
| Step 3 |
router
bgp
as-number
Example:
Device(config)# router bgp 40000
|
Enters router configuration mode for the specified routing process.
|
| Step 4 |
address-family
{ipv4 | ipv6}
[unicast]
Example:
Device(config-router)# address-family ipv4 unicast
|
Specifies the IPv4 or IPv6 address family and enters address family configuration mode.
|
| Step 5 |
neighbor
ip-address
aigp
Example:
Device(config-router-af)# neighbor 192.168.1.1 aigp
|
Enables send and receive of the AIGP attribute per neighbor.
|
| Step 6 |
end
Example:
Device(config-router-af)# end
|
Exits address family configuration mode and returns to privileged EXEC mode.
|
Configuring BGP Accumulated IGP
SUMMARY STEPS1.
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
DETAILED STEPS| | Command or Action | Purpose |
|---|
| Step 1 |
enable
Example:
|
Enables privileged EXEC mode.
|
| Step 2 |
configure
terminal
Example:
Device# configure terminal
|
Enters global configuration mode.
|
| Step 3 |
router
bgp
as-number
Example:
Device(config)# router bgp 40000
|
Enters router configuration mode for the specified routing process.
|
| Step 4 |
address-family
{ipv4 | ipv6}
[unicast]
Example:
Device(config-router)# address-family ipv4 unicast
|
Specifies the IPv4 or IPv6 address family and enters address family configuration mode.
|
| Step 5 |
neighbor
ip-address
aigp
[send {cost-community community-id poi {igp-cost | pre-bestpath} [transitive]} | med]
Example:
Device(config-router-af)# neighbor 192.168.1.1 aigp send med
|
Translates the AIGP attribute to MED and attaches it to the route before advertising to legacy provider edge (PE) devices.
|
| Step 6 |
end
Example:
Device(config-router-af)# end
|
Exits address family configuration mode and returns to privileged EXEC mode.
|
Configuration Examples for BGP Accumulated IGP
Example: Configuring AIGP Metric Value
The following is a sample configuration for originating prefixes
with the accumulated internal gateway protocol (AIGP) metric attribute:
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
Example: Enabling Send and Receive for an AIGP Attribute
The following example shows how to enable AIGP send and receive capability in address family configuration mode:
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)# exit
Example: Configuring BGP Accumulated IGP
In the following example, the device belongs to autonomous system 65000 and is configured to send the cost-community attribute to its neighbor at IP address 172.16.70.23:
Device# configure terminal
Device(config)# router bgp 65000
Device(config-router)# address-family ipv4 multicast
Device(config-router-af)# neighbor 172.16.70.23 aigp send cost-community 100 poi igp-cost transitive
Device(config-router-af)# exit
In the following example, the device belongs to autonomous system 65000 and is configured to send the MED attribute to its neighbor at IP address 172.16.70.23:
Device# configure terminal
Device(config)# router bgp 65000
Device(config-router)# address-family ipv4 multicast
Device(config-router-af)# neighbor 172.16.70.23 aigp send med
Device(config-router-af)# exit
Additional References for BGP Accumulated IGP
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Feature
Information for BGP Accumulated IGP
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1 Feature
Information for BGP Accumulated IGP|
Feature
Name
|
Releases
|
Feature
Information
|
|---|
|
BGP
Accumulated IGP
|
15.4(2)S
|
The BGP
Accumulated IGP feature is an optional nontransitive Border Gateway Protocol
(BGP) path attribute. The attribute type code for the accumulated interior
gateway protocol (AIGP) attribute is assigned by the IANA. The value field of
the AIGP attribute is defined as a set of type, length, value (TLV) elements.
The AIGP TLV contains the AIGP metric.
The
following commands were introduced:
aigp,
aigp send
cost-community,
aigp send
med,
bgp bestpath aigp
ignore,
set
aigp-metric
|
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