IPv6: Route Control - Fundamental Default On-Demand Routing
Fundamentals of On-Demand Routing Although the configuration of static routes is simple in a hub router such as the one in Figure 12-1, many network administrators still see static routes as administratively undesirable. The difficulty is not so much adding routes as new stub networks are brought online, as it is remembering to remove routes when stub networks or stub routers are taken offline. Beginning with IOS 11.2, Cisco offers a proprietary alternative for hub routers called On-Demand Routing (ODR). With ODR, a hub router can automatically discover stub networks while the stub routers still use a default route to the hub. ODR conveys address prefixesthat is, only the network portion of the addressrather than the entire addressso VLSM is supported. And because only minimal route information is traversing the link between the stub and hub routers, bandwidth is conserved. ODR is not a true routing protocol. It discovers information about stub networks but does not provide any routing information to the stub routers. The link information is conveyed by a data-link protocol and, therefore, does not go further than from the stub router to the hub router. However, as a case study will show, ODR-discovered routes can be redistributed into dynamic routing protocols. Example 12-1 shows a route table containing ODR entries. The table shows that the administrative distance is 160; the metric of the routes is 1. Because ODR routes are always from a hub router to a stub router, the metric (hop count) will never be more than 1. The routes also show that VLSM is supported. Example 12-1. This route table shows several ODR entries. Router#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF intN1 - OSPF NSSA external type 1, N2 - OSPF NSSA external E1 - OSPF external type 1, E2 - OSPF external type 2, E i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - c U - per-user static route, o - ODR Gateway of last resort is not set 192.168.1.0/24 is variably subnetted, 3 subnets, 2 masks o 192.168.1.40/30 [160/1] via 192.168.1.37, 00:00:27, Seri C 192.168.1.36/30 is directly connected, Serial0 C 192.168.1.192/27 is directly connected, Ethernet1 o 192.168.3.0/24 [160/1] via 192.168.1.37, 00:00:27, Serial0 192.168.4.0/24 is variably subnetted, 2 subnets, 2 masks o 192.168.4.48/29 [160/1] via 192.168.1.37, 00:00:27, Seri o 192.168.4.128/27 [160/1] via 192.168.1.37, 00:00:27, Ser Router# The transport mechanism for ODR routes is Cisco Discovery Protocol (CDP), a proprietary data link protocol that gathers information about neighboring network devices.  Example 12-2 shows the type of information collected by CDP.  CDP runs not only on routers but also on Cisco switches and access servers. Example 12-2. CDP collects information about neighboring Cisco network devices. Bumble#show cdp neighbors detail
Device ID: P8R1 Entry address(es): IP address: 10.131.223.226 Platform: Cisco 2620, Capabilities: Router Interface: Serial0/0.708, Port ID (outgoing port): Serial0/0.8Version : Cisco Internetwork Operating System Software IOS (tm) C2600 Software (C2600-J1S3-M), Version 12.3(6), RELEAS Copyright (c) 1986-2004 by Cisco Systems, Inc. Compiled Wed 11-Feb-04 19:24 by kellythw advertisement version: 2
Device ID: Blathers Entry address(es): IP address: 192.168.3.2 Platform: cisco 2610, Capabilities: Router Interface: Serial0/0.1, Port ID (outgoing port): Serial0/0.2 Holdtime : 122 sec Version : Cisco Internetwork Operating System Software IOS (tm) C2600 Software (C2600-J1S3-M), Version 12.3(10a), RELE ) Copyright (c) 1986-2004 by Cisco Systems, Inc. Compiled Fri 22-Oct-04 20:43 by kellythw advertisement version: 2
CDP runs on any media that supports the subnetwork access protocol (SNAP), which means that ODR also depends on SNAP support. Although CDP is enabled by default on all interfaces of all Cisco devices running IOS 10.3 and later, ODR support begins with IOS 11.2. Theconfiguration case study will show that ODR is configured on the hub router only; however, the stub routers must run IOS 11.2 or later for the hub router to discover their attached networks.