The match Commands for Policy Routing with Route Maps

The following match commands are used to determine whether the packet is one that is to be policy-routed, as opposed to being sent on its merry way. If so, it is sent down a different path, typically one less traveled.

Table 10-14

The match command is as follows: match {actions}

The match commands used in policy routing are summarized in Table 10-14.

match Commands Used in Policy Routing

Command match ip address [access-list-number I name] [...access-list-number I name]

match length min max

Description

This states the number or name of a standard or extended access list that will be used to examine incoming packets. The access list is used to match a packet with characteristics defined in it. If multiple access lists are specified, matching any one will result in a match.

This command is used to define the criteria based on the Layer 3 length of the packet.

The min parameter states the minimum inclusive length of the packet allowed for a match.

The max parameter states the maximum inclusive length of the packet allowed for a match.

In this way, interactive traffic that is time-sensitive, such as SNA traffic tunneled in IP, can be sent on a dedicated route.

Table 10-15

The following set commands are used after the match criteria has been satisfied. Whereas the match parameter determines whether the packet will be policy-routed, the set parameter determines how the packet is to be policy-routed.

The set command is as follows: set {actions}

The set commands used in policy routing are summarized in Table 10-15.

set Commands Used in Policy Routing

Command Description set default interface type number [...type number]

set interface type number [...type number]

If the routing table has no explicit route for the destination network of the packet, this set provides a list of default outbound interfaces. The packet being considered for policy routing is routed to the available outbound interface in the list of specified default interfaces.

If there is a route for the destination network of the packet in the routing table, this set provides a list of outgoing interfaces through which to route the packets. If more than one interface is specified, then the first functional outgoing interface is used.

This command has no effect and is ignored if the packet is a broadcast or is destined to an unknown address. This is because no explicit route for the destination of the packet was found in the routing table.

continues

Table 10-15 set Commands Used in Policy Routing (Continued)

Command

Description

set ip default next-hop ip

If the routing table has no explicit route for the destination network of

address [...ip address]

the packet, this set provides a list of default next-hop routers. The

packet being considered for policy routing is routed to the available

next hop in the list. This must be the address of an adjacent router.

set ip next hop ip address

If there is a route for the destination network of the packet in the

[...ip address]

routing table, this set provides a list of next-hop routers to which to

forward the packet. If more than one next hop is specified, then the

first available next-hop router is used. This must be the address of an

adjacent router.

set ip precedence

This is used to set the precedence bits in the Type of Service field of

precedence

the IP header of the matched packet. This determines the IP

precedence in the IP packets.

set ip tos type-of-service

This is used to set the IP TOS value in the Type of Service field of the

IP header.

The set commands can be used in conjunction with each other.

NOTE The match and set commands to configure a route map for redistribution are equally confusing.

You will be delighted to hear that they are not covered in the course, and it is unlikely that they will appear on the exam.

Having configured the route map, it must be called into service. Until it is called, it has no power.

The command used to recruit the services of the router map to an incoming interface follows:

ip policy route-map map-tag map-tag is the name of the route map to use for policy routing. This must match a map tag specified by a route-map command.

NOTE Policy-based routing is configured on the incoming interface that receives the packets, not on the outgoing interface from which the packets are sent.

Example 10-6 shows a sample of this configuration.

Example 10-6 Calling a Route Map into Service interface serial 0 ip policy route-map soupspoon !

route-map soupspoon match ip address 201.14.222.18

set ip next-hop 191.5.6.11

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