RE:Commands explaination of route map,set ip next hop

[ambivert skill]

Hello Experts,

Please let me know the reason of using following commands

1.route-map NAT permit 10

2.match ip address 101

3.set ip next-hop 192.168.18.20

Thank You

Ambivert Skill

[vmiller]

test the contents of access list 101,

if the address of the packet matches anything in access list 101

     set its next hop to 192.168.18.20

otherwise

     ignore it.

[Richard Burts · Accepted solution]

My colleague has done a good job of explaining what the route map does. But my reading of the original post suggests that the question is not so much what does it do but is about why are we doing this. So let me suggest this explanation about why:

the syntax of the route map suggests that it is configured as part of Policy Based Routing. (the name suggests that it is associated with Network Address Translation but the syntax suggests that it is associated with PBR). I suggest that if you look through the configuration and find where the route map is applied, that you will find that there is somewhere a statement of ip policy route-map NAT. This will confirm that the route map is used as part of PBR.

HTH

Rick

[ambivert skill]

Thanks to both of you for educating the concept , Richard yes you are right in my configuration of core switch it is mentioned "ip policy route-map NAT" under the SVI Interface of Public VLAN , actually we have ISA server on which NATing is enabled and all of our traffic comes on internal ethernet interface of ISA and after NATing it moves out from external interface of proxy server to my core switch on Gi0/1 --> SVI Int --> Gi0/2 int --> ASR and to internet.

Richard thanks for your wonderful explaination can you please guide me what is PBR and what is purpose of command

"route-map NAT permit 10"

Regards

Ambivert Skill

[vmiller]

PBR is the acronym for policy based routing. This enables the designer to modify the behavior of traffic based on definitions in a route map (policy) rather than the dynamic routing table.

What does the rest of the route map contain? All we have here is the routemap name and intial permit statement.

[Richard Burts]

Ambivert Skill

As my colleague explains normal routing uses the destination address and looks into the routing table for the best path to that destination. Policy Based Routing (or PBR) allows you to specify routing logic which over rides the logic of the routing table and allows you to make decisions about how to route a packet based on other criteria (such as matching an access list).

PBR uses a route map and part of your question is about the mechanics and the syntax of the route map. So let me explain:

The first line of a route map will have 4 elements which are

1) the command route-map

2) a variable which you make up and which identifies the route map. In your example the name is NAT

3) a keyword either permit or deny

4) a sequence number which you choose. In your example it is 10. The purpose of the sequence number is to allow the route map to have multiple instances which may define different conditions.

The route map will have the beginning line and may have one or more lines with match statements and may have one or more lines with set statements.

The example in your original post has one instance of the route map. It uses access list 101 to identify a certain set of traffic and specifies that the next hop for that traffic should be 192.168.18.20 (which we assume is not the next hop that would be found for the destination in the routing table).

It is possible that the route map might have a second instance which might look something like this

route-map NAT permit 20

match ip address 120

set ip next-hop 192.168.22.35

HTH

Rick