Policy Routing

Understanding Administrative Distance

When you use more than one routing protocol for example, when you redistribute you need to be aware of administrative distances. An administrative distance is a priority assigned to a route based on its routing protocol. When a router receives the same route from more than one protocol, which one should it use Should it, for example, use the route heard from RIP or the one heard from OSPF Because RIP and OSPF have completely different metrics, the router cannot decide by comparing metrics, so...

Default Routing and Classful Behavior

When you use a default route to reach a subnet of a connected major net, you must ensure that the router is configured with the ip classless global config command. Without ip classless, the router takes a classful posture and problems might surface. Consider the scenario depicted in Figure 3-12. In the network pictured in Figure 3-12, Router A is classful (configured with no ip classless) and has a default route pointing to Router B as the gateway of last resort. Everything in Router A's...

Controlling Redistribution Loops with Route Filters

When you combine redundant paths with mutual redistribution, you can encounter problems with redistribution loops. Take for example, the situation in Figure 3-7. Figure 3-7 A Redistribution Loop (Route Feedback) Figure 3-7 A Redistribution Loop (Route Feedback) Figure 3-7 shows two points along the IGRP-RIP boundary where redistribution is occurring. Both Router J and Router K are performing mutual redistribution between RIP and 1GRP. Care must be taken to ensure that a route redistributed from...

Uring Passive Interfaces

A common way of controlling routing information is to make an interface passive. A passive interface is a silenced interface an interface on which you deliberately suppress the advertising of routing updates. You might want to do this in certain situations. For example, for security reasons you might have to block routing updates sent to a particular department or company because the updates reveal the topology of your network. In another case, when redistributing from one routing protocol to...

Originating a Default Route with IGRP

As with RIP, you can configure a default route on an IGRP router like this ip route 0.0.0.0 0.0.0.0 192.168.202.1 The default route 0.0.0.0 is used locally by the router however, it cannot be advertised to any other router with IGRP. IGRP does not propagate 0.0.0.0 even if you redistribute it into IGRP with the redistribute static command. To make IGRP originate a default route and advertise it to others, configure IGRP as follows (this also works for EIGRP) router igrp ICC redistribute static...

Originating a Default Route with RIP

It's easy, sometimes dangerously easy, to eonfigure a router to originate a default route with RIP. All you need to do is configure the default route itself and RIP takes care of the rest. RIP automatically advertises the default out all RIP-enabled interfaces. Consider the example network depicted in Figure 3-11. Gateway of last resort (192.168.202.1) In the network in Figure 3-11, Router F has a default route to the Internet. The following is Router F's RIP configuration ip route 0.0.0.0...

Understanding Eigrp Auto Summarization

By default, EIGRP automatically summarizes across major net boundaries. Often, this is nothing to worry about however, when you deploy discontiguous subnets, as EIGRP supports, you might have to disable EIGRP auto-summarization. Consider the following scenario with auto-summarization enabled (see Figure 3-13). Assume EIGRP is the only routing protocol. Fig u re 3-13 EIGRP Auto-Summarization In the network depicted in Figure 3-13, major net 172.20.0.0 is broken up and separated by major net...

Originating a Default Route with OSPF

Like IGRP and EIGRP, OSPF does not automatically propagate a default route. To originate a default route with OSPF, configure OSPF with the default-information originate command router ospf 10 network 172.18.43.0 0.0.0.255 area 0 < other network commands> default-information originate metric 20 metric-type 1 ip route 0.0.0.0 0.0.0.0 172.18.44.2 where ip route 0.0.0.0 0.0.0.0 172.18.44.2 configures the default route and gateway of last resort. The command default-information originate metric...

Configuring OSPF Summarization During Redistribution

Unlike EIGRP, OSPF maintains a separate process for summarizing redistributed routes. This is called external route summarization. Consider the scenario depicted in Figure 3-16. Fig u re 3-16 OSPE External Route Sutnmarization Fig u re 3-16 OSPE External Route Sutnmarization Router A redistributes RIP into OSPF. Normally, the redistribution injects 16 routes, each with a 24 mask, into area 0. Instead of doing that. Router A can summarize the 16 routes and inject just a single route 172.20.16.0...