Lone Wolf Networks
7Dec/160

Cisco – Define default GW based on source

Was recently working on a project for a client where the client needed to route traffic from a specific network over a T1 and send the rest of the traffic out the default gateway of the router.  In order to accomplish the task I configured policy based routing on the 2911 as follows:

interface GigabitEthernet0/0.4

encapsulation dot1q 4

ip address 10.2.1.1 255.255.255.0

ip policy route-map NoDSL

access-list 101 permit 10.2.1.0 0.0.0.255 any

route-map NoDSL 10

match ip address 101

set ip default next-hop 172.16.1.29

Cisco has a pretty good example configuration white paper: http://www.cisco.com/c/en/us/support/docs/ip/ip-routed-protocols/47121-pbr-cmds-ce.html

The route map policy didn't seem to apply correctly, as traffic was hitting the default route instead of my defined next hop.  I enabled debugging and found the issue.  Debug output was showing the following:

R1#debug ip policy

R1#debug ip packet 101 detail

*May 26 14:45:21.359: IP: s=10.2.1.99 (GigabitEthernet0/0.4), d=64.4.4.1, len 84, FIB policy match

*May 26 14:45:21.359: IP: s=10.2.1.99 (GigabitEthernet0/0.4), d=64.4.4.1, len 84, FIB policy rejected(explicit route) - normal forwarding

The output showed me where the problem was.  In the Cisco example document, when used with a dynamic routing protocol the command to override the next-hop should omit the "default" statement (the reason is explained below).  I changed the route-map to the following:

route-map NoDSL 10

match ip address 101

set ip next-hop 172.16.1.29

Traffic began to flow as expected and matched the policy I had created.

This is an excellent policy routing reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_pi/configuration/15-mt/iri-15-mt-book/iri-pbr-default-nexthop-route.html

The applicable quote from this document:

The set ip next-hop and set ip default next-hop commands are similar but have a different order of operation. Configuring the set ip next-hop command causes the system to first use policy routing and then use the routing table. Configuring the set ip default next-hop command causes the system to first use the routing table and then the policy-route-specified next hop.

Posted by Jason Hower

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