416 CHAPTER 31: ROUTING POLICY CONFIGURATION
router’s address and so on. The match criteria can be set beforehand and then
apply them to a routing policy for route distribution, reception and redistribution.
Filters Routing protocols can use six filters: ACL, IP prefix list, AS path ACL, community
list, extended community list and routing policy.
ACL
ACL involves IPv4 ACL only. When defining an ACL, you can specify IP addresses
and prefixes to match destinations or next hops of routing information.
For ACL configuration, refer to “Configuring an Advanced IPv4 ACL” on page
844.
IP prefix list
IP prefix list plays a role similar to ACL, but it is more flexible than ACL and easier
to understand. When an IP prefix list is applied to filtering routing information, its
matching object is the destination address of routing information. Moreover, you
can specify the gateway option to indicate that only routing information
advertised by certain routers will be received.
An IP prefix list is identified by name. Each IP prefix list can comprise multiple
items, and each item, which is identified by an index number, can specify a
matching range in the network prefix format. The index number indicates the
matching sequence of items in the IP prefix list.
During matching, the router compares the packet with the items in the ascending
order. If one item is matched, the IP prefix list filter is passed, and the packet will
not go to the next item.
AS-path
AS path is only applicable to BGP. There is an AS-path field in the BGP packet. An
AS path list specifies matching conditions according to the AS-path field.
Community list
Community list only applies to BGP. The BGP packet contains a community
attribute field to identify a community. A community list specifies matching
conditions based on the community attribute.
Extended community list
Extended community list (extcommunity-list) applies to BGP only. It involves two
attributes: Route-Target extcommunity for VPN, Source of Origin extcommunity.
An extcommunity-list specifies matching conditions according to the two
attributes.
Routing policy
A routing policy is used to match against some attributes in given routing
information and modify the attributes of the information if match conditions are
satisfied. It can reference the above mentioned filters to define its own match
criteria.
A routing policy can comprise multiple nodes, which are in logic OR relationship.
Each node is a match unit, and the system compares each node to a packet in the
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