Boundary

Router D1 in Figure 8-12 would redistribute the routes. Notice that the interface between D1 and D2 has an IP address of 10.10.1.0. This indicates that if you had the same Enhanced IGRP process running across the entire network, with the default behavior of auto-summary activated, all the subnets of the 131.108.0.0 network would have been advertised as 131.108.0.0/16 routing entry, which is the major network class B.

Now, assume that in the network shown in Figure 8-12, there are different routing protocols and you must redistribute OSPF into Enhanced IGRP. The configuration for D1 would be as follows:

router eigrp 1 network 10.0.0.0 redistribute ospf 1 default-metric 100000 1 1 1 1

Show ip route at D2 for network 131.108.0.0:

D EX 131.10.1.0 [170/409600] via 10.10.1.1, 00:29:21, Serial0/3 D EX 131.10.2.0 [170/409600] via 10.10.1.1, 00:29:22, Serial0/3 D EX 131.10.30.0 [170/409600] via 10.10.1.1, 00:29:22, Serial0/3

D EX 131.10.252.0 [170/409600] via 10.10.1.1, 00:29:22, Serial0/3 D EX 131.10.253.0 [170/409600] via 10.10.1.1, 00:29:22, Serial0/3

D EX

131.10.254.0 [170/409600] via 10.10.1.1, 00:29:22, Serial0/3

Because these are all external routes into Enhanced IGRP, they are not summarized. To summarize all of them into one update across the serial line on router D1, there are two options: You could list this network under the Enhanced IGRP process, or you could use the ip summary-address command.

For the first option, the configuration commands on D1 would be as follows:

router eigrp 1 network 10.0.0.0 network 131.10.0.0 redistribute ospf 1 default-metric 100000 1 1

For the second option, the configuration commands on D1 would be as follows:

interface serial 0

ip address 10.10.1.1 255.255.255.0

ip summary-address eigrp 1 131.108.0.0 255.255.0.0

Now, when you perform a show ip route on D2, you will find a single route to network 131.108.0.0:

D 131.10.0.0/16 [90/409600] via 10.10.1.1, 00:29:22, Serial0/3

Whenever you use the summary-address command in Enhanced IGRP, the protocol follows two courses of action:

1. Creates a null route that matches the summary-address network and mask entry.

2. Limits the query propagation.

If the summary null route was not created by router D1 in its routing table, and router X in Figure 8-12 wants to send traffic to subnet 131.108.17.0/24, which does not exist, it would check its routing table and find a summary route via Enhanced IGRP to network 131.108.0.0/16.

This is the longest prefix the router has in its routing table. Router X would forward the packet to router D2, which would check its routing table and find a route to network 131.108.0.0/16 via D1. D2 would then forward the packet to D1. D1 does not have a subnet 131.108.0.0/17 in its routing table at this point, so D1 checks for the next closest prefix and finds 0.0.0.0/0 in its routing table via D2. Next, D1 forwards the packet back to D2. This ping-pong effect will continue until the TTL expires. With the null route in D1's routing table, a packet forwarded to D1 will be dropped by D1, and the router will send an ICMP unreachable message back to the source.

On the local router in which you configured the summary-address command (D1, in this case), Enhanced IGRP creates a matching masked route with the summary-address command to the null 0 interface. This action avoids routing loops. If you perform a show ip route on D1, the following route will appear:

You may wonder: What are the advantages of the null 0 route, and how does it prevent routing loops? Using the example network shown in Figure 8-12 again, this time assume that router D2 is the Internet connection and that you are sending a default route from D2. When you perform a show ip route on D1, you will see the following entry in the routing table:

Gateway of last resort is 10.10.1.2 to network 0.0.0.0

This indicates that if the router does not have a specific route to a destination, it will use the default route or it will use the router that is the gateway of last resort to reach that particular destination. Recall that the gateway of last resort is based on the concept that if the router does not have a specific routing entry to a destination, it uses the default route of 0.0.0.0 as the last resort to reach that destination.

For example, if a router is trying to reach a destination of 140.10.10.0/24 and does not find a route to this destination, it will determine whether it has a shorter prefix to the major network 140.10.0.0/16. If the major network is not there, then the router looks for a CIDR route of 140.0.0.0/8. If it does not find any entry that covers the destination it is trying to reach, the router will use the default route as its last resort.

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