Foundation Summary

     

The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam.

The characteristics of route maps are summarized in the following list:

• A route map has a list of criteria, stated with the match statement.

• A route map can change packets or routes that are matched by using the set statement.

• A collection of route map statements that have the same route map name are considered one route map.

• The route map will stop as soon as a match is made, just like an access list does.

• Within a route map, each route map statement is numbered with sequence numbers and, therefore, can be edited individually.

• Route maps can use IP standard or extended access lists to establish policy-based routing.

  - A standard IP access list can be used to specify match criteria for the source address of a packet.

  - Extended access lists can be used to specify match criteria based on source and destination addresses, application, protocol type, TOS, and precedence.

• The match route map configuration commands are used to define the conditions to be checked.

• The set route map configuration commands are used to define the actions to be followed if there is a match.

• A route map can contain logical ANDs as well as logical ORs.

• The sequence number is used to specify the order in which conditions are checked. Thus, if there are two statements in a route map named BESTEST, one with sequence 5 and the other with sequence 15, sequence 5 is checked first. If there is no match for the conditions in sequence 5, then sequence 15 will be checked.

The following characterize the operation of route map statements:

• The route map statements used for policy-based routing can be marked as permit or deny .

• Only if the statement is marked as permit and the packet meets the match criteria will the set commands be applied.

• The statements in a route map correspond to the lines of an access list. Specifying the match conditions in a route map is similar to specifying the source and destination addresses and masks in an access list.

• The statements in the route map are compared to the route or packet to see if there is a match. The statements are examined in turn from the top, like in an access list. The single match statement can contain multiple conditions. At least one condition in the match statement must be true. This is a logical OR.

• A route map statement can contain multiple match statements. All match statements in the route map statement must be considered true for the route map statement to be considered matched. This is a logical AND.

The route-map command syntax is shown here:

 Router(config)#  route-map   map-tag  [{  permit   deny  }  sequence-number  ] 

Table 18-8 describes the syntax options available for the route-map command.

Table 18-8. The route-map Command Options

Command	Description
map-tag	This is the name of the route map. This name is used to reference the route map when applied as an interface configuration command for policy-based routing or when called by the routing process in redistribution.
permit deny	(Optional) If the match criteria are met for this route map and permit is specified, the packet is forwarded as defined by the set actions. If the match criteria are not met and permit is specified, the next route map with the same map tag is tested . If there are no match criteria specified, but the packets or routes are permitted, then all packets or routes are set as specified. If there is no set statement, but the packets or routes are permitted, then all packets or routes that match the criteria are permitted. If a packet passes none of the match criteria for the set of route maps sharing the same name, it is sent to the normal routing process to be routed by destination. (Optional) If the match criteria are met for the route map and deny is specified, the packet is sent to the normal routing process, and no further route maps sharing the same map tag name will be examined.
sequence-number	(Optional) The sequence number indicates the position that a new route map will have in the list of route map statements already configured with the same name.

The match commands used in policy-based routing and redistribution are summarized in Table 18-9.

Table 18-9. The match Commands Used in Policy-Based Routing

Command	Description
match interface (IP)	Distributes any routes that have their next hop out one of the interfaces specified.
match ip address [ access-list-number name ] [... access-list-number name ]	This states the number or name of a standard or extended access list that will be used to examine incoming packets. A standard IP access list is used to match criteria for the source address of the packet. An extended IP access list is used to specify criteria based on source and destination, application, protocol type, TOS, and precedence. If multiple access lists are specified, matching any one will result in a match.
match ip next-hop	Redistributes any routes that have a next-hop router address passed by one of the access lists specified.
match ip route-source	Redistributes routes that have been advertised by routers and access servers at the address specified by the access lists.
match length min max	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. Interactive traffic uses small packets, so the links could be dedicated by packet size , allowing file transfers using large packets to use a separate link so that the terminal sessions are not starved of resources.
match metric (IP)	Redistributes routes with the metric specified.
match route-type (IP)	Redistributes routes of the specified type.
match tag	Redistributes routes in the routing table that match the specified tags.

The set commands used in policy-based routing are summarized in Table 18-10.

Table 18-10. The set Commands Used in Route Maps

Command	Description
set automatic-tag	Automatically computes the tag value.
set default 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-based routing is routed to the available outbound interface in the list of specified default interfaces.
set interface type number [ ...type number ]	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 broad-cast or is destined to an unknown address. This is because no explicit route for the destination of the packet is found in the routing table.
set ip default next-hop ip- address [ ...ip-address ]	If the routing table has no explicit route for the destination network of the packet, this set provides a list of default next-hop routers. The packet being considered for policy-based 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 [ ...ip-address ]	If there is a route for the destination network of the packet in the 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 precedence	This is used to set the precedence bits in the Type of Service field of 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.
set level { level-1 level-2 level-1-2 stub-area backbone }	Used by IS-IS to determine the level of router to which the process should import routes. Also used by OSPF to state the type of area router to which routes should be imported.
set metric (BGP, OSPF, RIP)	Sets the metric value for a routing protocol.
set metric-type { internal external type-1 type-2 }	Sets the metric type for the destination routing protocol.
set tag tag-value	Sets a tag value of the destination routing protocol.

Figure 18-2 and Figure 18-3 summarize the logic used when route maps are applied to a router. Remember that in Figure 18-3, if there is no match or the match is a deny, the packet is not discarded but sent to the routing process to be routed by destination. If, however, the routing table has no entry for the destination, the packet will be dropped. This is not a function of route maps but rather the normal routing process.

Figure 18-2. Route Map Logic for Policy-Based Routing 1

Figure 18-3. Route Map Logic for Policy-Based Routing 2