1.2d Policy Based Routing

Policy based routing (PBR) is a mechanism by which we can route traffic in a manner other than that based on its destination.  PBR allows forwarding decision to be based on source, destination, protocol type (FTP, HTTP, etc. ) or incoming interface.

How does it work?

• Traffic criteria defined by a route-map match
  • Permit means to policy route
  • Deny means to use normal forwarding
  • match [interface | ip address | …]
• Action defined by route-map set
  • set ip [default] next-hop
  • set [default] interface
• Policy can be verified with…
  • debug ip policy

PBR can be applied to two types of traffic

• Incoming traffic
  • PBR applied at the link level
  • ip policy route-map [route-map]
• Locally originated traffic
  • PBR applied globally
  • ip local policy route-map [route-map]

PBR Caveats

• Most platforms can’t hardware accelerate PBR
• Platforms that do have strict criteria limits
• Outside of this PBR should be used sparingly

Configuration example

Define interesting traffic

ip access-list extended HOSTS8-HOSTS10-SUBNETS-TO-HOSTS9
 permit ip 155.1.8.0 0.0.0.255 155.1.9.0 0.0.0.255
 permit ip 155.1.10.0 0.0.0.255 155.1.9.0 0.0.0.255

Create route-map and set next-hop (or whatever you want)

route-map HOST8-HOST10-PBR permit 10
 match ip address HOSTS8-HOSTS10-SUBNETS-TO-HOSTS9
 set ip next-hop 155.1.0.3

Apply policy to appropriate interface

interface GigabitEthernet0/2
 ip policy route-map HOST8-HOST10-PBR