Configure BGP Route Properties

Configuration Condition

Before configuring BGP route properties, ensure that:

• BGP is enabled.
• BGP neighbors are configured and a session is set up successfully.

Configure the BGP Route Weight

In BGP routing, the first rule is to compare the weights of routes. The larger the weight of a route is, the higher the priority it has. The weight of a route is the local property of the device, and it cannot be transferred to other BGP neighbors. The value range of a route weight is 1-65535. By default, the weight of a route that has been learnt from a neighbor is 0, and the weights of all routes that are generated by the local device are all 32768.

Table 12-18 Configure the BGP route weight

Configure the MED Property of a BGP Route

Multi-Exit Discriminator (MED) properties are used to select the optimal route for the traffic that enters an AS. If the other routing conditions are the same and BGP learns several routes with the same destination from different EBGP neighbors, BGP select the route with the minimum MED value as the optimal ingress.

MED sometimes is also called external metric. It is marked as a "metric" in the BGP routing table. BGP advertises the MED properties of the routes that it has learnt from neighbors to IBGP neighbors, but BGP does not advertise the MED properties to EBGP neighbors. Therefore, MED properties are applicable to only adjacent ASs.

Configure BGP to allow comparing MEDs of neighbor routes from different ASs.

By default, BGP implements MED route selection only among the routes that are from the same AS. However, you can run the bgp always-compare-med command to let BGP ignore the limitation on the same AS in MED route selection.

Table 12-19 Configure BGP to allow comparing MEDs of neighbor routes from different ASs

Configure BGP to sort and select MEDs according to AS_PATH groups.

By default, BGP is not enabled to sort and select MEDs according to route AS_PATH groups. To enable the function, run the bgp deterministic-med command. In route selection, all routes are organized based on AS_PATHs. In each AS_PATH group, routes are sorted based MED values. The route with the minimum MED value is selected as the optimal route in the group.

Table 12-20 Configure BGP to sort and select MEDs according to AS_PATH groups

Configure to compare MEDs of routes in the local confederation.

By default, the MED values of EBGP routes from different ASs are not compared. The setting is valid for the EBGP routes of confederations. To enable comparison of MED values of routes of the local confederation, run the bgp bestpath med confed command.

Table 12-21 Configure BGP to compare MEDs of routes in the local confederation

Configure a route map to modify MED properties.

In transmitting and receiving routes, you can apply a route map to modify MED properties.

Table 12-22 Configure a route map to modify MED properties

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  In configuring a route map to modify an MED property, you can use the set metric command to modify the MED property. For details, refer to Routing Policy Tools-Technical Manual - set metric.
• After the neighbor attribute-unchanged command is configured, the MED properties of neighbors cannot be changed by the route map that is applied.

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Configure the Local-Preference Property of a BGP Route

Local-Preference properties are transferred only between IBGP neighbors. Local-Preference is used to select the optimal egress of an AS. The route with the maximum Local-Preference will be selected with priority.

The value range of Local-Preference is 0-4294967295. The larger the value is, the higher priority the route is. By default, the Local-Preference value of all the routes that are advertised to IBGP neighbors is 100. You can use the bgp default local-preference command or the route map to modify the Local-Preference property value.

Configure BGP to modify the default Local-Preference property.

Table 12-23 Configure BGP to modify the default local-preference property

Configure the route map to modify the Local-Preference property.

Table 12-24 Configure the route map to modify the local-preference property

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  In configuring a route map to modify the Local-Preference property, you can use the set local-preference command to modify the Local-Preference property. For details, refer to Routing Policy Tools-Technical Manual-set local-preference.

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Configure the AS_PATH Property of a BGP Route

Configure BGP to ignore AS_PATHs in route selection.

If the other conditions are the same, BGP selects the route with the shortest AS-PATH in route selection. To cancel route selection based on AS_PATHs, run the bgp bestpath as-path ignore command.

Table 12-25 Configure BGP to ignore AS_PATHs in route selection

Configure the number of local ASs that BGP allows to repeat.

To prevent routing loops, BGP checks the AS_PATH properties of the routes that are received from neighbors, and the routes containing the local AS number will be discarded. However, you can run the neighbor allowas-in command to allow the AS_PATH properties of the routes that the BGP receives to contain the local AS number, and you can configure the number of ASs that can be contained.

Table 12-26 Configure the number of local ASs that BGP allows to repeat

Configure BGP to remove the private AS number when advertising routes to neighbors.

In a large-scale BGP network, the AS_PATH properties of routes contain federation or community property. By default, BGP provides the private AS properties when it advertises routes to neighbors. To mask private network information, run the neighbor remove-private-AS command to remove the private AS number.

Table 12-27 Configure BGP to Remove the Private AS number when advertising routes to neighbors

Configure to check the validity of the first AS number of an EBGP route.

When BGP advertises a route to EBGP neighbors, it compresses the local AS number to the starting position of the AS_PATH, and the AS that advertises the route first is located at the end. Usually, the first AS of a route that EBGP receives must be the same as the neighbor AS number; otherwise, the route will be discarded.

Table 12-28 Configure to check the validity of the first AS number of an EBGP route

Configure a route map to modify AS_PATH properties.

BGP supports configuring a route map to modify AS_PATH properties. You can run the set as-path prepend command to add more routing properties so as to affect neighbor routing. In using the set as-path prepend function, first use the local AS to add AS_PATH. If you use another AS, the AS must be emphasized to prevent the AS from rejecting routes that are advertised to it.

Table 12-29 Configure a Route map to modify AS_PATH properties

• In configuring a route map to modify an AS_PATH property, you can use the set as-path prepend command to modify the AS_PATH property. For details, refer to Routing Policy Tools-Technical Manual-set as-path.

Configure the NEXT-HOP Property of a BGP Route

When BGP advertises routes to IBGP neighbors, it does not change the routing properties (including the NEXT-HOP property). When BGP advertises the routes that are learned from EBGP neighbors to IBGP neighbors, you can run the neighbor next-hop-self command to modify the next-hop property of the routes advertised to BGP neighbors to the local IP address. You can apply a route map to modify the next hop property.

Configure BGP to use the local IP address as the next hop of a route.

Table 12-30 Configure BGP to use the local IP address as the next hop of a route