Implementing Path Control Using Offset Lists
This section introduces offset lists and how to configure and verify path control using offset lists.
Using Offset Lists to Control Path Selection
An offset list is the mechanism for increasing incoming and outgoing metrics to routes learned via EIGRP or Routing Information Protocol (RIP). (Offset lists are only used for distance vector routing protocols.) Optionally, an offset list can be limited by specifying either an access list or an interface.
Configuring Path Control Using Offset Lists
To add an offset to incoming and outgoing metrics to routes learned via EIGRP or RIP, use the offset-list {access-list-number | access-list-name} {in | out} offset [interface-type interface-number] router configuration command, as explained in Table 5-2.
Table 5-2. offset-list Command
|
Parameter |
Description |
|
access-list-number | access-list-name |
Standard access list number or name to be applied. Access list number 0 indicates all access lists. If the offset value is 0, no action is taken. |
|
in |
Applies the access list to incoming metrics. |
|
out |
Applies the access list to outgoing metrics. |
|
offset |
Positive offset to be applied to metrics for networks matching the access list. If the offset is 0, no action is taken. |
|
interface-type interface-number |
(Optional) Interface type and number to which the offset list is applied. |
The offset value is added to the routing metric. An offset list that specifies an interface type and interface number is considered to be an extended list and takes precedence over an offset list that is not extended. Therefore, if an entry passes the extended offset list and a normal offset list, the offset of the extended offset list is added to the metric.
Figure 5-3 illustrates an example network in which an organization is using RIP and is connected to the Internet service provider (ISP) via edge Routers R4 and R5. A subset of routes is received from each of the edge routers. The metric between Routers R2 and R5 is smaller than the metric between Routers R2 and R4, because it is only one hop. However, this is very slow link. An offset list can be used on Router R2 so that it prefers the path toward the edge Router R4 for a specific set of destinations.
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Figure 5-3 An Offset List Can Be Used to Prefer a Faster Path.
A partial configuration of Router R2 is shown in Example 5-1. In this example, the offset-list 21 in 2 serial 0/0 command adds an offset of 2 to the metric of routes learned from interface serial 0/0 (connected to Router R5) that are permitted by access list 21. Access list 21 permits a specific set of routes (any in the 172.16.0.0/16 network) being learned from Router R5. This command is entered in RIP configuration mode on Router R2. This configuration results in the path toward Router R4 being considered better for the set of selected routes; R4 becomes the preferred way out toward the ISP for these routes.
Example 5-1. Offset List Configuration for Router R2 in Figure 5-3
router rip offset-list 21 in 2 serial 0/0 ! access-list 21 permit 172.16.0.0 0.0.255.255
Verifying Path Control Using Offset Lists
You can use the traceroute EXEC to verify that an offset list is affecting the path that traffic takes.
The routing table, viewed with the show ip route command, identifies the metrics for learned routes. You should compare these metrics to what was expected by the offset list configuration. For EIGRP, the EIGRP topology table can be examined using the show ip eigrp topology command. The topology table contains all routes learned from the router's EIGRP neighbors, and includes the metric information for those routes, including the best route and any other feasible routes that the router has learned about.
You can use debug commands, such as debug ip rip and debug ip eigrp, to view the real-time processing of incoming and outgoing RIP routing updates, to ensure that the metric is being processed appropriately.