Summarizations Effect on the Routing Table

Chapter 6 discussed several techniques used to avoid routing loops when dealing with redistribution. Summarization has these issues as well, and there is a unique way to prevent them in summarization. Refer to Figure 7-4 for another view of the network you are summarizing.

Figure 7-4 Summarization Sample Network i

Mutual Redistribution s0 ^ asbr

BP01 Frame Relay EIGRP

Figure 7-4 Summarization Sample Network

BP01 Frame Relay EIGRP

Because Routers Trinity and Tank performed the summarization when a router is trying to reach one of the summarized networks, they go to the router that is advertising the summary. Notice the highlighted entry in Trinity's routing table in Example 7-8.

Example 7-8 Trinity's Routing Table

Trinity#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route

Gateway of last resort is not set is variably subnetted, 3 subnets, 2 masks C is directly connected, Serial0/0

C is directly connected, Serial0/0

O E2 [110/100] via, 01:03:06, FastEthernet0/0 is variably subnetted, 6 subnets, 2 masks R [120/1] via, 00:00:23, Serial0/0

O is a summary, 01:10:18, Null0

R [120/1] via, 00:00:23, Serial0/0

R [120/1] via, 00:00:24, Serial0/0

R [120/1] via, 00:00:24, Serial0/0

O E2 [110/100] via, 00:00:19, FastEthernet0/0 is variably subnetted, 3 subnets, 2 masks C is directly connected, FastEthernet0/0

C is directly connected, Loopback0

O [110/2] via, 01:03:10, FastEthernet0/0

O IA [110/2] via, 01:03:10, FastEthernet0/0 O IA [110/2] via, 01:03:10, FastEthernet0/0 O IA [110/2] via, 01:03:10, FastEthernet0/0 Trinity#

Rather curious to see the entire summary routed to nullO? NullO is a fictitious interface that causes the router to drop into the bit bucket any information that is destined to it. This begs the question: Why is it present in Trinity's routing table? In addition, the following entry is also present in Router Tank's routing table for the summary it is advertising:

O is a summary, 00:02:30, Null0

These entries are placed into the routing table to prevent routing loops. To understand how this could happen, consider that this route entry is not present in the routing table and a failure occurs, making network unreachable. Other OSPF routers do not know and do not care. Because this route entry has the summary, it continues to advertise that summary, thus making it appear that the route to the network is still valid. Add the presence of a default route in area 10, which points to Router Neo.

420 Chapter 7: Summarization with OSPF

NOTE These null routes are automatically entered only in Cisco IOS Software Release 12.1(6) and later. Previously, you would have had to enter a static discard route manually in configuring a router.

Now, try and reach network from Router Apoc. The packets are going to go to Router Neo, which on checking its routing table, sees the summary route and forwards the packets to Router Trinity, the ASBR advertising the summary. When Router Trinity receives these packets, it knows that this network is down, so it uses the default route and sends the packets back to Router Neo, which sends them back to Trinity. The result is one big routing loop.

However, when summarizing in OSPF, you have the summary route to Null0 which, regardless of the up/down status of a network that is part of the summary, is always active. When out packets come from Router Neo destined to, they are routed to Null0, not back to Router Neo, thus avoiding a routing loop.

The next section looks at a real-world example of how you can take a network and subnet it effectively while ensuring that each summarization is possible.

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