Auto Summary and Route Aggregation

The IOS is optimized to perform routing as fast as possible. Most of the Layer 3 routing performance improvement in the brief history of routers has been through improved algorithms; many times those improved algorithms later have been implemented in hardware to provide additional latency improvements. Although these improvements have been a great benefit, it is typically true that any algorithm that searches a list will run more quickly if the list is short, compared to searching a similar list that is long. Auto summary and route aggregation (also known as route summarization) are two IOS features that reduce the size of the IP routing table.

Auto summarization is a routing protocol feature that operates by this rule:

When advertised on an interface whose IP address is not in network X, routes about subnets in network X will be summarized and advertised as one route. That route will be for the entire Class A, B, or C network X.

Auto summary is a feature of RIP-1 and IGRP that cannot be disabled. For RIP-2 and EIGRP, auto summary can be enabled or disabled. As usual, an example makes the concept much clearer. Consider Figure 6-11, which shows two networks in use: 10.0.0.0 and 172.16.0.0. Seville has four (connected) routes to subnets of network 10.0.0.0. Example 6-14 lists the output of a show ip route command on Albuquerque, as well as RIP-2 debug ip rip output.

Figure 6-11 Auto Summarization

Example 6-14 Albuquerque's Routing Table When Seville Is Summarizing

Albuquerque#debug ip rip

02:20:42:

RIP: sending v2 update to 224.0.0.9 via Serial0.2 (172

16.1.251)

02:20:42:

172.16.2.0/24 -> 0.0.0.0, metric 1, tag 0

02:20:42:

RIP: sending v2 update to 224.0.0.9 via Ethernet0 (172

16.2.251)

02:20:42:

172.16.1.0/24 -> 0.0.0.0, metric 1, tag 0

02:20:42:

10.0.0.0/8 -> 0.0.0.0, metric 2, tag 0

02:20:46:

RIP: received v2 update from 172.16.1.253 on Serial0.2

02:20:46:

10.0.0.0/8 -> 0.0.0.0 in 1 hops

Albuquerque#

Albuquerque#undebug all

All possible debugging has been turned off

Albuquerque#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, 2 subnets

C 172.16.1.0 is directly connected, Serial0.2

C 172.16.2.0 is directly connected, Ethernet0

R 10.0

.0.0/8 [120/1] via 172.16.1.253, 00:00:09, Serial0.2

Notice as highlighted in Example 6-14 that Albuquerque's received update on Serial0.2 from Seville advertises only about the entire Class A network 10.0.0.0/8 because auto summary is enabled on Seville (by default). The IP routing table lists just one route to network 10.0.0.0. This works fine, as long as network 10.0.0.0 is contiguous. Consider Figure 6-12, where

Yosemite also has subnets of network 10.0.0.0 but has no connectivity to Seville other than through Albuquerque.

Figure 6-12 Auto Summarization Pitfalls

IP subnet design traditionally has not allowed discontiguous networks. A contiguous network is a single Class A, B, or C network for which all routes to subnets of that network pass through only other subnets of that same single network. Discontiguous networks refer to the concept that, in a single Class A, B, or C network, there is at least one case in which the only routes to one subnet pass through subnets of a different network. An easy analogy for residents in the United States is the familiar term contiguous 48, referring to the 48 states besides Alaska and Hawaii. To drive to Alaska from the contiguous 48, for example, you must drive through another country (Canada, for the geographically impaired!), so Alaska is not contiguous with the 48 states—in other words, it is discontiguous.

Figure 6-12 breaks that rule. In this figure, there could be a PVC between Yosemite and Seville that uses a subnet of network 10.0.0.0, but that PVC may be down, causing the discontiguous network. The temporarily discontiguous network can be overcome with the use of a routing protocol that transmits masks because the rule of discontiguous subnets can be ignored when using a routing protocol that transmits masks. Consider the routing updates and routing table on Albuquerque in Example 6-15, where auto summarization is disabled on all routers.

Example 6-15 Albuquerque's Routing Table When Seville is Not Summarizing

debug ip

rip

RIP protocol

debugging is on

Albuquerque#

02:48:58

RIP

: received v2 update

from 172

16.1.253 on Serial0.2

02:48:58

10.1.7.0/24 -> 0.0

0.0 in

1

hops

02:48:58

10.1.6.0/24 -> 0.0

0.0 in

1

hops

02:48:58

10.1.5.0/24 -> 0.0

0.0 in

1

hops

02:48:58

10.1.4.0/24 -> 0.0

0.0 in

1

hops

Example 6-15 Albuquerque's Routing Table When Seville is Not Summarizing (Continued)

02

49

14

RIP:

received v2 update

from 172.16.3.252 on Serial0.1

02

49

14

10.1.11.0/24 -> 0.

0.1

3.0 in 1 hops

02

49

14

10.1.10.0/24 -> 0.

0.1

3.0 in 1 hops

02

49

14

10.1.9.0/24 -> 0.0

.0

0 in 1 hops

02

49

14

10.1.8.0/24 -> 0.0

.0

0 in 1 hops

02

49

16

RIP:

sending v2 update

to

224.0.0.9 via Serial0.1

(172.16.3.251)

02

49

16

172.16.1.0/24 -> 0

.0.

0.0, metric 1, tag 0

02

49

16

172.16.2.0/24 -> 0

.0.

0.0, metric 1, tag 0

02

49

16

10.0.0.0/8 -> 0.0.

0.0

3, metric 2, tag 0

02

49

16

RIP:

sending v2 update

to

224.0.0.9 via Serial0.2

(172.16.1.251)

02

49

16

172.16.2.0/24 -> 0

.0.

0.0, metric 1, tag 0

02

49

16

172.16.3.0/24 -> 0

.0.

0.0, metric 1, tag 0

02

49

16

10.0.0.0/8 -> 0.0.

0.0

, metric 2, tag 0

02

49

16

RIP:

sending v2 update

to

224.0.0.9 via Ethernet 0

(172.16.2.251

02

49

16

172.16.1.0/24 -> 0

.0.

0.0, metric 1, tag 0

02

49

16

172.16.3.0/24 -> 0

.0.

0.0, metric 1, tag 0

02

49

16

10.0.0.0/8 -> 0.0.

0.0

, metric 2, tag 0

Albuquerque#no debug all

All possible debugging has been turned off

Albuquerque#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, 3 subnets

C

172

.16.

1.

0 is directly connected,

Serial0.2

C

172

.16.

2.

0 is directly connected,

Ethernet0

C

172

.16.

3.

0 is directly connected,

Serial0.1

10.0.0

.0/24

is subnetted, 8 subnets

R

10.

1.11

.0

[120/1] via 172.16.3.252

, 00

00:15

Serial0.1

R

10.

1.10

.0

[120/1] via 172.16.3.252

, 00

00:15

Serial0.1

R

10.

1.9.

0

[120/1] via 172.16.3.252,

00

0

0:

15,

Serial0.1

R

10.

1.8.

0

[120/1] via 172.16.3.252,

00

0

0:

15,

Serial0.1

R

10.

1.7.

0

[120/1] via 172.16.1.253,

00

0

0:

03,

Serial0.2

R

10.

1.6.

0

[120/1] via 172.16.1.253,

00

0

0:

03,

Serial0.2

R

10.

1.5.

0

[120/1] via 172.16.1.253,

00

0

0:

03,

Serial0.2

R

10.

1.4.

0

[120/1] via 172.16.1.253,

00

0

0:

03,

Albuquerque#

Notice as highlighted in Example 6-15 that the routing updates include the individual subnets. Therefore, Albuquerque can see routes to all subnets of network 10 and can route packets to the correct destinations in Seville and Yosemite. With auto summary enabled, Albuquerque would think that both Seville and Yosemite had an equal-metric route to network 10.0.0.0; some packets would be routed incorrectly.

Route summarization (also called route aggregation) works like auto summarization, except that there is no requirement to summarize into a Class A, B, or C network. Consider the same network in Figure 6-12. Albuquerque has eight routes to subnets of network 10.0.0.0; four of those routes are learned from Seville. Consider the subnet, broadcast, and assignable addresses in each of the subnets, as shown in Table 6-15.

Table 6-15 Route Aggregation Comparison of Subnet Numbers

Subnet

Mask

Broadcast

Assignable Addresses

10.1.4.0

255.255.255.0

10.1.4.255

10.1.4.1 to 10.1.4.254

10.1.5.0

255.255.255.0

10.1.5.255

10.1.5.1 to 10.1.5.254

10.1.6.0

255.255.255.0

10.1.6.255

10.1.6.1 to 10.1.6.254

10.1.7.0

255.255.255.0

10.1.7.255

10.1.7.1 to 10.1.7.254

Now consider the concept of a subnet 10.1.4.0, with mask 255.255.252.0. In this case, 10.1.4.0/22 (same subnet, written differently) would have a subnet broadcast address of 10.1.7.255 and assignable addresses of 10.1.4.1 to 10.1.7.254. Because 10.1.4.0/22 happens to include all the assignable addresses of the original four subnets, a single route to 10.1.4.0/22 would be just as good as the four separate routes, assuming that the next-hop information would be the same for each of the original four routes.

Route aggregation is simply a tool used to tell a routing protocol to advertise a single, larger subnet rather than the individual smaller subnets. In this case, the routing protocol would advertise 10.1.4.0/22 rather than the four individual subnets. Albuquerque's routing table will then be smaller. EIGRP and OSPF are the only interior IP routing protocols to support route aggregation.

Route summarization of the subnets off Seville is shown in Example 6-16. Still using the network of Figure 6-12, the routers are all migrated to EIGRP. Example 6-16 shows the EIGRP configuration on Albuquerque, EIGRP configuration on Seville, and the resulting IP routing table on Albuquerque. (Yosemite is migrated to EIGRP as well; the configuration is not shown because the example shows only aggregation by Seville.)

Example 6-16 Route Aggregation Example Using EIGRP

On Seville:

Router eigrp 9

Network 10.0.0.0

Network 172.16.0.0

1

interface serial 0.1

point-to-point

ip address 172.16.1

253 255.255.255.0

frame-relay interface-dlci 901

Example 6-16 Route Aggregation Example Using EIGRP (Continued)

ip summary-address eigrp 9 10.1.4.0 255.255.252.0

On Albuquerque:

Router eigrp 9

Network 172.16.0.0 No auto-summary

Albuquerque#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, 3 subnets C 172.16.1.0 is directly connected, Serial0.2

C 172.16.2.0 is directly connected, Ethenet0

C 172.16.3.0 is directly connected, Serial0.1

10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks D 10.1.11.0/24 [120/1] via 172.16.3.252, 00:00:15, Serial0.1

D 10.1.10.0/24 [120/1] via 172.16.3.252, 00:00:15, Serial0.1

D 10.1.9.0/24 [120/1] via 172.16.3.252, 00:00:15, Serial0.1

D 10.1.8.0/24 [120/1] via 172.16.3.252, 00:00:15, Serial0.1

D 10.1.4.0/22 [90/2185984] via 172.16.1.253, 00:00:58, Serial0.2

The ip summary-address interface subcommand on Seville's serial 0.1 interface is used to define the superset of the subnets that should be advertised. Notice the route in Albuquerque's routing table, which indeed shows 10.1.4.0/22, rather than the four individual subnets.

When summarizing, the superset of the original subnets could actually be smaller than the Class A, B, or C network; larger than the network; or exactly matched to a network. For instance, 192.168.4.0, 192.168.5.0, 192.168.6.0, and 192.168.7.0 could be summarized into 192.168.4.0/22, which represents four consecutive Class C networks. Summarization when the summarized group is a set of networks is sometimes called supernetting.

Table 6-16 lists the features for summarization of the interior IP routing protocols.

Table 6-16 Route Aggregation Comparison of Subnet Numbers

Table 6-16 lists the features for summarization of the interior IP routing protocols.

Table 6-16 Route Aggregation Comparison of Subnet Numbers

Routing Protocol

Auto Summary Enabled?

Auto Summary Disabled?

Route Aggregation?

RIP Version 1

Yes, by default

Not allowed

No

IGRP

Yes, by default

Not allowed

No

RIP Version 2

Yes, by default

Allowed via configuration

No

Table 6-16 Route Aggregation Comparison of Subnet Numbers (Continued)

Auto Summary

Auto Summary

Routing Protocol

Enabled?

Disabled?

Route Aggregation?

Enhanced IGRP

Yes, by default

Allowed via

Yes

configuration

OSPF

No, but can do

Yes

Yes

equivalent with

aggregation

0 0

Post a comment