Configuration Example 1

In Figure 4, a route map is used to determine the weight for routes learned by router Chicago.

Figure 4: Route Map Filtering

Access list 1 permits any route whose AS-path attribute includes 32 or 837. This condition permits routes that originate in (or pass through from elsewhere) AS 32 or AS 837. When these routes are advertised through AS 451 and AS 17 to router Chicago, instance 1 of route map 1 matches such routes and sets their weight to 25, overriding the neighbor weight set for updates received from 10.2.2.4.

Access list 2 permits any route whose AS-path attribute indicates that it originates in AS 74. When these routes are advertised through AS 837 and AS 32 to router Chicago, instance 1 of route map 2 matches such routes and sets their weight to 175, overriding the neighbor weight set for updates received from 10.5.5.2.

The following example configures router Chicago:

The result of this configuration is that router Chicago prefers routes learned through router Boston (weight 150) over routes learned through router NY (weight 50), except that:

• Router Chicago prefers routes learned via router NY that passed through AS 837 or AS 32 (weight 50) over the same routes learned via router Boston (weight 25 according to route map 1).
• Router Chicago prefers routes originating in AS 74 learned via router NY that passed through AS 837 and AS 32 (weight 175 according to route map 2) over the same routes learned via router Boston (weight 150).

access-list

• Use to define an IP access list to permit or deny routes based on the prefix.
• Each access list is a set of permit or deny conditions for routes based on matching a route's prefix.
• A zero in the wildcard mask means that the corresponding bit in the address must be exactly matched by the route. A one in the wildcard mask means that the corresponding bit in the address does not have to be matched by the route.
• Use the neighbor distribute-list command to apply the access list to routes received from or forwarded to a neighbor.
• Use the log keyword to log an Info event in the ipAccessList log whenever an access list rule is matched.
• Example

  host1(config)#access-list bronze permit ip host any 228.0.0.0 0.0.0.255

• Use the no version to delete an IP access list (no other options specified), the specified entry in the access list, or the log for the specified access list or entry (by specifying the log keyword).
• See access-list.

default-information originate

• Use to enable RIP, OSPF, or BGP to advertise a default route (0.0.0.0/0) that exists in the IP routing table.
• If you specify the always option for OSPF, OSPF generates a default route, if it does not exist in the IP routing table and advertises it.
• Use to generate a default route to an IS-IS domain.
• Example

  host1(config-router)#default-information originate

• Use the no version to disable advertisement of the default route.
• See default-information originate.

ip as-path access-list

• Use to define an AS-path access list to permit or deny routes based on the AS path.
• Each access list is a set of permit or deny conditions for routes based on matching a route's AS path to a regular expression. If the regular expression matches the representation of the AS path of the route as an ASCII string, the permit or deny condition applies. The AS path does not contain the local AS number.
• The AS path allows substring matching. For example, the regular expression 20 matches AS path = 20 and AS path = 100 200 300, because 20 is a substring of each path. To disable substring matching and constrain matching to only the specified attribute string, place the underscore (_) metacharacter on both sides of the string; for example, _20_.
• Use the neighbor filter-list command to apply the AS-path access list. You can apply access-list filters to inbound and outbound BGP routes. You can assign weights to routes matching the AS-path access list.
• Example

  host1(config)#ip as-path access-list 1 permit ^\(
  

• Use the no version to remove the AS-path access list; all entries that belong to this list are removed.
• See ip as-path access-list.

ipv6 access-list

• Use to define an IPv6 access list to permit or deny routes based on the prefix.
• Each access list is a set of permit or deny conditions for routes based on matching a route's prefix.
• Use the neighbor distribute-list command to apply the access list to routes received from or forwarded to a neighbor.
• Use the log keyword to log an Info event in the ipAccessList log whenever an access list rule is matched.
• Example

  host1(config)#ipv6 access-list bronze deny 1::1/16 any
  

• Use the no version to delete an IPv6 access list (no other options specified), the specified entry in the access list, or the log for the specified access list or entry (by specifying the log keyword).
• See ipv6 access-list.

neighbor distribute-list

• Use to filter routes to selected prefixes as specified in an access list. Distribute lists are applied only to external peers.
• Use the in keyword to apply the list to inbound routes (inbound policy). Use the out keyword to apply the list to outbound routes (outbound policy).
• Besides using distribute lists to filter BGP advertisements, you can do the following:
  • Use AS-path filters with the ip as-path access-list and the neighbor filter-list commands
  • Use route map filters with the route-map and the neighbor route-map commands
• Example

  host1:vr1(config-router)#neighbor group1 distribute-list list1 in

• Use the no version to disassociate the access list from a neighbor.
• See neighbor distribute-list.

neighbor filter-list

• Use to assign an AS-path access list to matching inbound or outbound routes.
• Use the in keyword to apply the list to inbound routes (inbound policy). Use the out keyword to apply the list to outbound routes (outbound policy).
• You can specify an optional weight value with the weight keyword to assign a relative importance to incoming routes that match the AS-path access list.
• Access list values can be in the range 0–65535.
• Example

  host1:vr1(config-router)#neighbor group2 filter-list list2 out

• Use the no version to disassociate the access list from a neighbor.
• See neighbor filter-list.

neighbor prefix-list

• Use to assign an inbound or outbound prefix list.
• If you specify a BGP peer group by using the peer-group-name argument, all the members of the peer group inherit the characteristic configured with this command unless it is overridden for a specific peer.
• Use the in keyword to assign the prefix list to incoming routes (inbound policy)
• Use the out keyword to assign the prefix list to outgoing routes (outbound policy); you cannot configure a member of a peer group to override the inherited peer group characteristic for outbound policy
• Example

  host1(config-router)#neighbor 192.168.1.158 prefix-list seoul19 in

• Use the no version to remove the prefix list.
• See neighbor prefix-list.

neighbor prefix-tree

• Use to assign an inbound or outbound prefix tree.
• If you specify a BGP peer group by using the peer-group-name argument, all the members of the peer group inherit the characteristic configured with this command unless it is overridden for a specific peer.
• Use the in keyword to assign the prefix tree to incoming routes (inbound policy)
• Use the out keyword to assign the prefix tree to outgoing routes (outbound policy); you cannot configure a member of a peer group to override the inherited peer group characteristic for outbound policy
• Example

  host1(config-router)#neighbor 192.168.1.158 prefix-tree newyork out

• Use the no version to remove the prefix tree.
• See neighbor prefix-tree.

redistribute

• Use to redistribute routes from one routing domain to another routing domain.
• Example

  host1(config)#router bgp 100

  host1(config-router)#neighbor 192.56.10.2 remote-as 200

  host1(config-router)#redistribute static

  host1(config-router)#exit

  host1(config)#ip route 155.30.0.0 0.0.255.255

• Use the no version to end redistribution of information.
• See redistribute.

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