3Com Switch 8800 Advanced Software V5 Configuration Guide

244 CHAPTER 25: ROUTING POLICY CONFIGURATION
ACL
ACL involves IPv4 ACL and IPv6 ACL. 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 “ACL Overview” on page 801.
IP prefix list
IP prefix list involves IPv4 and IPv6 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. For gateway option information,
refer to
“RIP Configuration” on page 269 and “OSPF Configuration” on page 301.
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 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 list
AS path list 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.
The Source of Origin extcommunity attribute, which is the application in the
source routing feature, is not supported currently.
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