Filters - 3Com MSR 50 Series Configuration Manual

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HAPTER OUTING OLICY ONFIGURATION

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 and IPv6 ACL. When defining an ACL, you can specify IP
addresses and subnets to match destinations or next hops of routing information.
For ACL configuration, refer to "Configuring ACLs" on page 1881.
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 971 and "OSPF Configuration" on page 917.
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.
The filtering relation among items is logical OR. 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 ACL
AS path ACL is only applicable to BGP. There is an AS-path field in the BGP packet.
An AS path ACL 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. Each node is a match unit, and the
system compares each node to a packet in ascending order of node sequence
numbers.