Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 358)

330 Multi-Service IronWare Routing Configuration Guide
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Configuration considerations
Policy-Based Routing (PBR) allows you to use ACLs and route maps to selectively modify and route
IP packets in hardware. The ACLs classify the traffic. Route maps that match on the ACLs set
routing attributes for the traffic.
A PBR policy specifies the next hop for traffic that matches the policy. Using standard ACLs with
PBR, you can route IP packets based on their source IP address. With extended ACLs, you can route
IP packets based on all of the match criteria in the extended ACL.
You can configure the Brocade device to perform the following types of PBR based on a packet’s
Layer 3 and Layer 4 information:
• Select the next-hop gateway.
• Send the packet to the null interface (null0).
When a PBR policy has multiple next hops to a destination, PBR selects the first live next hop
specified in the policy that is up. If none of the policy's direct routes or next hops is available, the
packets are forwarded as per the routing table.
Configuration considerations
The configuration considerations are as follows:
• A PBR policy on an interface takes precedence over a global PBR policy.
• You cannot apply PBR on a port if that port already has inbound ACLs, inbound ACL-based rate
limiting, or TOS-based QoS.
• The number of route maps that you can define is limited by the system memory. When a route
map is used in a PBR policy, the PBR policy uses up to 200 instances of a Layer 3 route map,
up to 5 ACLs in a matching policy of each route map instance.
The following two conditions can cause more than 200 Layer 3 route-map instances to be
used.
1. If one or more of first 200 instances have deny clause.
2. If the access-list used in the first 200 instances is not configured.
• ACLs with the log option configured should not be used for PBR purposes.
• PBR ignores implicit deny ip any any ACL entries, to ensure that for route maps that use
multiple ACLs, the traffic is compared to all the ACLs. However, if an explicit deny ip any any is
configured, traffic matching this clause will be routed normally using Layer 3 paths and will not
be compared to any ACL clauses that follow this clause.
• PBR always selects the first next hop from the next hop list that is up. If a PBR policy's next hop
goes down, the policy uses another next hop if available. If no next hops are available, the
device routes the traffic in the normal way.
• Any changes to route maps or ACL definitions will be effective immediately for the interfaces
where the PBR routemap is applied. There is no need to rebind. However, rebinding is required
if a change is made to an IPv6 ACL.
• If a PBR policy is applied globally, inbound ACLs, inbound ACL-based rate-limiting or TOS-based
QoS cannot be applied to any port on the device.
• If an IPv4 option packet matches a deny ACL filter with the option keyword, the packet will be
forwarded based on Layer-3 destination. If the ignore-options command is configured on the
incoming physical port, the packet will be forwarded based on its Layer-3 destination in
hardware, otherwise the packet will be sent to the CPU for software forwarding.