BSCI exam success, not to mention earning your CCNP, can come down to your OSPF route summarization skills. There are a few different commands and situations you need to be ready for, and one of these situations is the proper use of the "summary-address" command.
The summary-address command should be used on an ASBR in order to summarize routes that are being injected into the OSPF domain via redistribution. In the following example, four routes are being redisitributed into OSPF on R1, making R1 an ASBR.
interface Loopback16
ip address 16.16.16.16 255.0.0.0
!
interface Loopback17
ip address 17.17.17.17 255.0.0.0
!
interface Loopback18
ip address 18.18.18.18 255.0.0.0
!
interface Loopback19
ip address 19.19.19.19 255.0.0.0
R1(config)#router ospf 1
R1(config-router)#redistribute connected subnets
These four routes are seen on downstream router R2 as External Type-2, the default for routes redistributed into OSPF.
R2#show ip route ospf
O E2 17.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0
O E2 16.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0
O E2 19.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0
O E2 18.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0
To summarize networks learned by redistribution, use the OSPF command summary-address. You can probably do this summarization in your head, but do so before continuing with the lab.
R1(config)#router ospf 1
R1(config-router)#summary-address 16.0.0.0 252.0.0.0
Look at the change in R2's OSPF table.
R2#show ip route ospf
O E2 16.0.0.0/6 [110/20] via 172.12.123.1, 00:00:05, Serial0
The external routes have been successfully summarized. Note that the summary route is still marked as an E2 route.
There's an interesting route installed into R1's OSPF table as well.
R1#show ip route ospf
O 16.0.0.0/6 is a summary, 00:01:51, Null0
When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table. This helps to prevent routing loops. Any packets destined for the routes that have been summarized will have a longer match in the routing table....
C 17.0.0.0/8 is directly connected, Loopback17
C 16.0.0.0/8 is directly connected, Loopback16
C 19.0.0.0/8 is directly connected, Loopback19
C 18.0.0.0/8 is directly connected, Loopback18
O 16.0.0.0/6 is a summary, 00:03:10, Null0
O 12.0.0.0/6 is a summary, 00:07:53, Null0
.. and packets that do not match one of the summarized routes but do match the summary route will be dropped.
Showing posts with label summarize. Show all posts
Showing posts with label summarize. Show all posts
Thursday, December 25, 2008
Cisco CCNP / BSCI Exam Tutorial: EIGRP Route Summarization
Summarizing routes is a vital skill to learn to pass the BSCI exam and get one step closer to earning your CCNP. The actual binary conversions are only part of the test, though! You've got to know how to correctly apply the summary routes, and that differs from one protocol to the next. In the last few CCNP / BSCI tutorials, we've looked at using the "area range" and "summary-address" commands to perform OSPF route summarization. Today, we'll take a look at summarizing routes in EIGRP.
We'll use the following four loopback addresses in this example:
Loopback 16, 16.16.16.16 /32
Loopback 17, 17.17.17.17 /32
Loopback 18, 18.18.18.18 /32
Loopback 19. 19.19.19.19 /32
On R1, we'll place these four addresses into EIGRP AS 100.
R1(config-if)#router eigrp 100
R1(config-router)#network 16.16.16.16 0.0.0.0
R1(config-router)#network 17.17.17.17 0.0.0.0
R1(config-router)#network 18.18.18.18 0.0.0.0
R1(config-router)#network 19.19.19.19 0.0.0.0
R3 is an EIGRP neighbor of R1, and that router's EIGRP routing table now looks like this:
R3#show ip route eigrp
17.0.0.0/32 is subnetted, 1 subnets
D 17.17.17.17 [90/2297856] via 172.12.123.1, 00:00:29, Serial0
16.0.0.0/32 is subnetted, 1 subnets
D 16.16.16.16 [90/2297856] via 172.12.123.1, 00:00:36, Serial0
19.0.0.0/32 is subnetted, 1 subnets
D 19.19.19.19 [90/2297856] via 172.12.123.1, 00:00:08, Serial0
18.0.0.0/32 is subnetted, 1 subnets
D 18.18.18.18 [90/2297856] via 172.12.123.1, 00:00:22, Serial0
To perform manual route summarization, write out the network addresses in binary and then determine the point at which the addresses no longer have a bit in common. For these four addresses, it will be enough to write out the first octet in binary:
16 00010000
17 00010001
18 00010010
19 00010011
Working from left to right, the common bits are the first six bits - 000100xx. In decimal, this value is 16. The summary mask must be determined as well, and that value is derived from putting a "1" in the mask for each common bit. With the first six bits all set to one - 11111100 - the resulting mask is 252.0.0.0. The full summary address is 16.0.0.0 252.0.0.0.
In EIGRP, the summary address is actually configured on an interface, not under the routing process.
R1(config)#interface serial0
R1(config-if)#ip summary-address eigrp 100 16.0.0.0 252.0.0.0
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is down: summary configured
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is down: summary configured
02:40:16: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is up : new adjacency
02:40:17: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is up: new adjacency
There's an immediate side effect here that most books leave out. Your EIGRP adjacencies are going to come down after you configure this summary, but they should come back up quickly. The key word there is "should". If you configure EIGRP summary addresses on a production network, you may want to do this during non-peak hours. The timestamps on the above commands indicate that the adjacencies were down for about 27 seconds over the NBMA network. That's about 30 minutes in end-user time. ;)
Check R3's EIGRP routing table.
R3#show ip route eigrp
D 16.0.0.0/6 [90/2297856] via 172.12.123.1, 00:01:46, Serial0
The four summarized routes are no longer in the routing table, and they have been replaced by the summary route shown at the bottom of the routing table. Notice the mask is /5, which is prefix notation for 248.0.0.0.
Knowing how and why to summarize routes is a valuable skill, regardless of the protocol in use. But before you take the BSCI exam on your way to the CCNP, make sure you know how to perform summarization with all of the core protocols!
We'll use the following four loopback addresses in this example:
Loopback 16, 16.16.16.16 /32
Loopback 17, 17.17.17.17 /32
Loopback 18, 18.18.18.18 /32
Loopback 19. 19.19.19.19 /32
On R1, we'll place these four addresses into EIGRP AS 100.
R1(config-if)#router eigrp 100
R1(config-router)#network 16.16.16.16 0.0.0.0
R1(config-router)#network 17.17.17.17 0.0.0.0
R1(config-router)#network 18.18.18.18 0.0.0.0
R1(config-router)#network 19.19.19.19 0.0.0.0
R3 is an EIGRP neighbor of R1, and that router's EIGRP routing table now looks like this:
R3#show ip route eigrp
17.0.0.0/32 is subnetted, 1 subnets
D 17.17.17.17 [90/2297856] via 172.12.123.1, 00:00:29, Serial0
16.0.0.0/32 is subnetted, 1 subnets
D 16.16.16.16 [90/2297856] via 172.12.123.1, 00:00:36, Serial0
19.0.0.0/32 is subnetted, 1 subnets
D 19.19.19.19 [90/2297856] via 172.12.123.1, 00:00:08, Serial0
18.0.0.0/32 is subnetted, 1 subnets
D 18.18.18.18 [90/2297856] via 172.12.123.1, 00:00:22, Serial0
To perform manual route summarization, write out the network addresses in binary and then determine the point at which the addresses no longer have a bit in common. For these four addresses, it will be enough to write out the first octet in binary:
16 00010000
17 00010001
18 00010010
19 00010011
Working from left to right, the common bits are the first six bits - 000100xx. In decimal, this value is 16. The summary mask must be determined as well, and that value is derived from putting a "1" in the mask for each common bit. With the first six bits all set to one - 11111100 - the resulting mask is 252.0.0.0. The full summary address is 16.0.0.0 252.0.0.0.
In EIGRP, the summary address is actually configured on an interface, not under the routing process.
R1(config)#interface serial0
R1(config-if)#ip summary-address eigrp 100 16.0.0.0 252.0.0.0
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is down: summary configured
02:39:50: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is down: summary configured
02:40:16: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.2 (Serial0) is up : new adjacency
02:40:17: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor
172.12.123.3 (Serial0) is up: new adjacency
There's an immediate side effect here that most books leave out. Your EIGRP adjacencies are going to come down after you configure this summary, but they should come back up quickly. The key word there is "should". If you configure EIGRP summary addresses on a production network, you may want to do this during non-peak hours. The timestamps on the above commands indicate that the adjacencies were down for about 27 seconds over the NBMA network. That's about 30 minutes in end-user time. ;)
Check R3's EIGRP routing table.
R3#show ip route eigrp
D 16.0.0.0/6 [90/2297856] via 172.12.123.1, 00:01:46, Serial0
The four summarized routes are no longer in the routing table, and they have been replaced by the summary route shown at the bottom of the routing table. Notice the mask is /5, which is prefix notation for 248.0.0.0.
Knowing how and why to summarize routes is a valuable skill, regardless of the protocol in use. But before you take the BSCI exam on your way to the CCNP, make sure you know how to perform summarization with all of the core protocols!
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