Chapter 12. Link-State Protocols: Open Shortest Path First (OSPF)

EIGRP Summarization

Understanding EIGRP summarization and knowing how to effectively use it are absolutely vital to the design of large EIGRP networks. EIGRP scales very well, but when the number of routes starts to climb into the hundreds, extra care should be taken to control route propagation and the query range. As much as EIGRP is plug-and-play on small networks, it is not on large networks. The larger the network is, the more care should be taken to control how routes propagation.

Summarization provides two powerful enhancements to EIGRP. First, by lowering the number of routes in the route table, it lessens the number and size of the EIGRP advertisements. Second, and more importantly, it can limit the EIGRP query range.

Controlling the Query Range Through Summarization, Addressing Stuck in Active (SIA) Route Issues

Arguably one of the most common and complex problems facing large EIGRP networks is stuck in active (SIA) routes. A route becomes SIA when EIGRP is "actively" running computations for the route, and it doesn't stop. EIGRP will log multiple messages similar to the following:

 %DUAL-3-SIA: Route 192.168.1.16 Stuck-in-Active 

Most of the time, the route shows active because it is waiting for query to return from neighbor. There can be many reasons for this:

• The router is overutilized ” Edge routes can get flooded with query request and often don't have the CPU power to keep up with the request. Queries receive irregular replies, if any, and the route stays active.

• The router is having memory issues ” This can be compounded from a slow processor and the router having many items in its queues.

• Overutilized circuit ” EIGRP hellos might not be getting through, causing the neighbor to drop.

Two types of configurations can manifest the SIA situation:

• Most EIGRP networks have autosummary disabled. This is primarily because the IP addressing scheme has discontinuous subnets, so the query range is not bounded.

• Large Frame Relay networks have many remote sites coming into the same router, therefore, there are many EIGRP neighbors.

For example, in a Frame Relay network, if a single PVC goes inactive or a route starts flapping, it can cause a small EIGRP query storm . Figures 11-7 and 11-8 illustrate a common Frame Relay network and the query process.

Figure 11-7. EIGRP Network

Figure 11-8. EIGRP Query Storm

If the PVC is lost from canada_1 to mexico_1, the canada_1 router sends an EIGRP query message to all of its neighbors regarding the routes that it lost from the mexico_1 router. It is looking for a new feasible successor to the routes. In this case, the message goes to the swiss_1 and usa_1 routers. The mexico_1 router also sends a query message to all of its neighbors looking for a new feasible successor for the routes that it lost from canada_1. All routers in the EIGRP domain continue to issue queries to neighbors. The routes stay in an "active" state until EIGRP receives "replys" to the queries that it sent. If you scale this network to a router with an HSSI or T3 interface, it would be possible to have hundreds of PVCs on a single interface, and the loss of just a single PVC could generate hundreds or thousands of queries. Fortunately, summarization bounds the query process and is one of the most effective ways to control EIGRP query storms. A large EIGRP network without summarization is an SIA problem looking for an owner.

EIGRP Autosummarization

By default, EIGRP performs autosummarization in two situations:

• Autosummarization will occur at the major class boundary during redistribution from EIGRP into a classful routing protocol, such as IGRP or RIP. This type of summarization cannot be disabled.

• Autosummarization will occur at the major class boundary when the route is advertised out an interface that is on a different major class boundary. This summarization can be disabled with the command no auto-summary from the router(config-router) prompt.

EIGRP will not automatically summarize EIGRP external routes.

EIGRP routes that are summarized have an administrative distance of 90. In Figure 11-9, the internetwork has been modified from the previous example, adding two additional networks to the canada_2 router.

Figure 11-9. EIGRP Autosummarization

With autosummarization enabled, EIGRP on the canada_2 router advertises two summary routes to canada_1. The routes 182.16.3.0/24 and 182.16.4.0/24 are advertised as 182.16.0.0/16. The routes 10.1.1.0/24 and 10.1.2.0/30 are advertised at their natural class boundary with a route of 10.0.0.0/8. It is important to note that EIGRP summarizes the route only when advertising out an interface that is in a different class. For example, if the network between canada_1 and canada_2 is 10.1.3.0/24, the 10's network would not be summarized; only the 182.16.x.x networks would. Example 11-23 lists the route table of canada_1, highlighting the summarized routes.

Example 11-23 Route Table of canada_1 with Summarized Routes

 canada_1#  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, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/24 is subnetted, 5 subnets D       172.16.5.0 [90/2195456] via 172.16.1.5, 01:06:41, Serial0.1 D       172.16.6.0 [90/2195456] via 172.16.2.6, 01:08:01, Serial0.2 C       172.16.1.0 is directly connected, Serial0.1 C       172.16.2.0 is directly connected, Serial0.2 C       172.16.3.0 is directly connected, TokenRing0      192.168.4.0/32 is subnetted, 1 subnets D EX    192.168.4.1 [170/2195456] via 172.16.2.6, 01:07:46, Serial0.2  D    10.0.0.0/8 [90/304128] via 172.16.3.3, 00:51:27, TokenRing0   D    182.16.0.0/16 [90/304128] via 172.16.3.3, 00:51:27, TokenRing0  192.168.3.0/32 is subnetted, 1 subnets D EX    192.168.3.1 [170/2195456] via 172.16.2.6, 01:07:46, Serial0.2 canada_1# 

As helpful as EIGRP summarization might appear on the surface, it has serious drawbacks on most modern networks. It essentially makes a classless routing protocol enforce the discontinuous subnets rule at the major bit boundaries. When EIGRP forms a summary route to advertise, it also forms a route to null for all the networks in that summary. For example, the canada_2 router will form three routes to null for each major class network that the router has an interface in. The route to null will discard any packets that this router does not have an explicit route to. Example 11-24 demonstrates autosummary null routes displayed by the show ip route command.

Example 11-24 EIGRP Autosummary Null Routes

 canada_2#  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, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks D       172.16.5.0/24 [90/2211584] via 172.16.3.1, 00:12:02, TokenRing0 D       172.16.6.0/24 [90/2211584] via 172.16.3.1, 00:12:02, TokenRing0 D       172.16.0.0/16 is a summary, 00:12:06, Null0 D       172.16.1.0/24 [90/2185984] via 172.16.3.1, 00:12:02, TokenRing0 D       172.16.2.0/24 [90/2185984] via 172.16.3.1, 00:12:02, TokenRing0 C       172.16.3.0/24 is directly connected, TokenRing0      192.168.4.0/32 is subnetted, 1 subnets D EX    192.168.4.1 [170/2211584] via 172.16.3.1, 00:12:02, TokenRing0      10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks C       10.1.2.0/30 is directly connected, Loopback32  D       10.0.0.0/8 is a summary, 00:12:06, Null0  C       10.1.1.0/24 is directly connected, Loopback31      182.16.0.0/16 is variably subnetted, 3 subnets, 2 masks C       182.16.4.0/24 is directly connected, Loopback21 C       182.16.3.0/24 is directly connected, Loopback20  D       182.16.0.0/16 is a summary, 00:12:07, Null0  192.168.3.0/32 is subnetted, 1 subnets D EX    192.168.3.1 [170/2211584] via 172.16.3.1, 00:12:03, TokenRing0 canada_2# 

• For autosummarization to work properly, discontiguous subnets at the major bit boundaries must be avoided at all costs. Unfortunately, on modern networks, something causes subnets to be deployed in places they shouldn't be, and EIGRP has forwarding problems. By disabling autosummarization with the no auto-summary command, the routes to null are not created and automatic summary routes are not forwarded. In place of autosummary, use manual summarization. Most engineers that we have worked with disable autosummarization when using EIGRP to prevent routes to the null interface.

EIGRP Manual Summarization or Route Aggregation

EIGRP manual summarization is critical to large EIGRP networks. It limits the EIGRP query and can significantly reduce the size of the routing table. There are essentially two ways to deploy manual summarization:

• Advertise an summary address or aggregate address with the following interface command:

    ip summary-address eigrp   as_number summary_address address_mask  

• Advertise a default route with the following interface command:

    ip summary-address eigrp   as_number 0.0.0.0 0.0.0.0.  

This command causes only the default route to be advertised; all other routing updates are suppressed.

One of the powerful functions of EIGRP is the capability to advertised multiple summary routes and default routes on different interfaces. The EIGRP network in Figure 11-10 is under the same autonomous system, and autosummary has been disabled on all the routers. In this model, EIGRP is configured to advertise a default route out the s0.2 port on the canada_1 router to the usa_1 router. The canada_1 router also advertises two summary routes, 182.0.0.0/8 and 10.0.0.0/8, out the s0.1 interface to the mexico_1 router.

Figure 11-10. EIGRP Manual Summarization

Example 11-25 lists the configuration of canada_1 router performing manual summarization.

Example 11-25 Manual Summarization on canada_1 Serial Interfaces

 interface Serial0  no ip address  encapsulation frame-relay  no ip mroute-cache ! interface Serial0.1 point-to-point  ip address 172.16.1.1 255.255.255.0  ip summary-address eigrp 2001 182.0.0.0 255.0.0.0  Manual summarization   ip summary-address eigrp 2001 10.0.0.0 255.0.0.0  frame-relay interface-dlci 110 ! interface Serial0.2 point-to-point  ip address 172.16.2.1 255.255.255.0  ip summary-address eigrp 2001 0.0.0.0 0.0.0.0  Advertise a default route only  frame-relay interface-dlci 130 ! interface TokenRing0  ip address 172.16.3.1 255.255.255.0  ring-speed 16 ! router eigrp 2001  network 172.16.0.0  no auto-summary ! 

Example 11-26 lists the route table of the mexico_1 router and the usa_1 router. Notice how the routes are summarized. The usa_1 router receives only a default route 0.0.0.0, and the gateway of last resort is set.

Example 11-26 Route Tables of mexico_1 and usa_1 with Summarization Applied

 mexico_1#  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, * - candidate default        U - per-user static route, o - ODR        T - traffic engineered route Gateway of last resort is not set      172.16.0.0/24 is subnetted, 5 subnets C       172.16.5.0 is directly connected, Ethernet0 D       172.16.6.0 [90/2707456] via 172.16.1.1, 00:34:11, Serial0.1 C       172.16.1.0 is directly connected, Serial0.1 D       172.16.2.0 [90/2681856] via 172.16.1.1, 00:34:11, Serial0.1 D       172.16.3.0 [90/2185984] via 172.16.1.1, 00:34:11, Serial0.1 D    192.168.4.0/24 [90/2835456] via 172.16.1.1, 00:34:11, Serial0.1 D    10.0.0.0/8 [90/2313984] via 172.16.1.1, 00:34:11, Serial0.1  Summary Route D    192.168.3.0/24 [90/2835456] via 172.16.1.1, 00:34:11, Serial0.1 D    182.0.0.0/8 [90/2313984] via 172.16.1.1, 00:34:11, Serial0.1  Summary Route mexico_1# _______________________________________________________________________ usa_1#  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, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is 172.16.2.1 to network 0.0.0.0 D    192.168.3.0/24 [90/409600] via 172.16.6.7, 00:45:52, Ethernet0 D    192.168.4.0/24 [90/409600] via 172.16.6.7, 00:45:52, Ethernet0      172.16.0.0/24 is subnetted, 2 subnets C       172.16.6.0 is directly connected, Ethernet0 C       172.16.2.0 is directly connected, Serial0.1 D*   0.0.0.0/0 [90/2185984] via 172.16.2.1, 00:44:54, Serial0.1 usa_1# 

NOTE

In Cisco IOS Software 12.0(4)T, an administrative distance can be added to the summary address to alter the default admin distance of 90.