Users Guide

Access Control Lists 717
Policy-Based Routing
In contemporary inter-networks, network administrators often need to
implement packet forwarding/routing according to specific organizational
policies. Policy-Based Routing (PBR) exactly fits this purpose. Policy-Based
Routing provides a flexible mechanism to implement solutions where
organizational constraints dictate that traffic be routed through specific
network paths. PBR does not affect route redistribution that occurs via
routing protocols.
PBR is a true routing policy solution. The packet TTL is decremented in
PBR-routed packets. The destination MAC is rewritten in PBR-routed
packets. ARP lookups are sent when required for unresolved next-hop
addresses.
Configuring PBR consists of installing a route-map with match and set
commands and then applying the corresponding route-map to the routing
VLAN interface. IP routing must be enabled on the interfaces by assigning IP
addresses to the VLAN interfaces, assigning the VLANs to physical interfaces,
and enabling IP routing globally.
Packet Classification
Route-maps may specify multiple packet attributes in match statements.
These attributes can be matched through a “match” clause based on length
of the packet or a “match” clause linked with up to 16 ACLs.
The match attributes listed below for each ACL type indicate the criteria
used to classify Layer-3 routed traffic for PBR. At least one of the listed
attributes must be present in the ACL of the given type:
• VLAN tag (implicitly added)
• MAC access list (match mac-list)
– Source MAC address
– 802.1p priority
• IP access list (match ip address)
– Source or destination IP address
• Protocol ID field in the IP header
• L3 packet length in the IP header (match length)