Basic Uses of Route Maps

 

Route maps can be used for both redistribution and for policy routing. They are also used frequently in large-scale Border Gateway Protocol (BGP) implementations . Although redistribution has been covered extensively in previous chapters, this chapter introduces the topic of policy routing.

Policy routes are nothing more than sophisticated static routes. Whereas static routes forward a packet to a specified next hop based on the destination address of the packet, policy routes forward a packet to a specified next hop based on the source of the packet. Policy routes can also be linked to extended IP access lists so that routing may be based on such things as protocol types and port numbers . Like a static route, a policy route influences the routing only of the router on which it is configured.

Note

policy routing


Figure 14.1 shows an example of a typical policy routing application. AbnerNet is connected to two Internet service providers via router Dogpatch. AbnerNet's corporate policy dictates that some users' Internet traffic should be sent via ISP 1 and other users' Internet traffic should be sent via ISP 2. If either ISP should become unavailable, the traffic normally using that provider will be sent to the other provider. A policy route at Dogpatch can distribute Internet traffic in accordance with local policy. The distribution of traffic might be based on subnet, specific user, or even user applications.

Figure 14.1. Policy routing allows traffic from AbnerNet to be routed to one of its two Internet service providers based on parameters such as source address, source/destination address combinations, packet size , application-level ports, or even packet length.

graphics/14fig01.gif

Figure 14.2 shows another use for policy routing. One of the systems on the right watches for invasion forces from the planet Mongo while the other stores back copies of Dilbert comic strips . Policy routes can be configured to route the critical traffic from the Mongo System to Flash_G over the FDDI link and to route the lower-priority Dilbert traffic over the 56K links. Or vice versa.

Figure 14.2. Policy routing allows high-priority traffic from the Mongo System to be routed over the FDDI link while low-priority traffic from the Dilbert System is routed over the 56K links.

graphics/14fig02.gif

table 14.1 and table 14.2 show the match and set commands that can be used with redistribution, and table 14.3 and table 14.4 show the match and set commands that can be used with policy routing.

Table 14.1. Match commands that can be used with redistribution.

Command

Description

match interface type number [ type number ]

Matches routes that have their next hop out one of the interfaces specified.

match ip address { access-list-number name } [ access-list-number name ]

Matches routes that have a destination address specified by one of the access lists.

match ip next-hop { access-list-number name } [ access-list-number name ]

Matches routes that have a next-hop router address specified by one of the access lists.

match ip route-source { access-list-number name } [ access-list-number name ]

Matches routes that have been advertised by routers at the addresses specified in the access lists.

match metric metric-value

Matches routes with the specified metric.

match route-type { internal external [ type-1 type-2 ] level-1 level-2 }

Matches OSPF, EIGRP, or IS-IS routes of the specified type.

match tag tag-value [ tag-value ]

Matches routes with the specified tags.

Table 14.2. Set commands that can be used with redistribution.

Command

Description

set level { level-1 level-2 level-1-2 stub-area backbone }

Sets the IS-IS level or the OSPF area into which a matched route is to be redistributed.

set metric { metric-value bandwidth delay reliability loading mtu }

Sets the metric value for a matched route.

set metric-type { internal external type-1 type-2 }

Sets the metric type for a matched route being redistributed into IS-IS or OSPF.

set next-hop next-hop

Sets the next-hop router address for a matched route.

set tag tag-value

Sets a tag value for a matched route.

Table 14.3. Match commands that can be used with policy routing.

Command

Description

match ip address { access-list-number name } [ access-list-number name ]

Matches a packet with the characteristics specified in the standard or extended access lists.

match length min max

Matches the level 3 length of a packet.

Table 14.4. Set commands that can be used with policy routing.

Command

Description

set default interface type number [ type number ]

Sets the outgoing interface for matched packets when there is no explicit route to the destination.

set interface type number [ type number ]

Sets the outgoing interface for matched packets when there is an explicit route to the destination.

set ip default next-hop ip-address [ ip-address ]

Sets the next-hop router address for matched packets when there is no explicit route to the destination.

set ip next-hop ip-address [ ip-address ]

Sets the next-hop router address for matched packets when there is an explicit route to the destination.

set ip precedence precedence

Sets the precedence bits in the Type of Service field of matched IP packets.

set ip tos type-of-service

Sets the TOS bits in the Type of Service field of matched packets.



Routing TCP[s]IP (Vol. 11998)
Routing TCP[s]IP (Vol. 11998)
ISBN: N/A
EAN: N/A
Year: 2004
Pages: 224

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