To pass your BSCI exam and earn your CCNP certification, you've got to master route summarization. When you get to the BSCI level, actually breaking the routes down into binary strings and performing summarization is second nature to you. (If it isn't, get some more practice!) What makes CCNP / BSCI route summarization more difficult is just keeping the different protocol summarization commands straight!
RIP and EIGRP both perform route summarization at the interface level with the ip summary-address command. In the following example, R2 is running RIP and was sending four routes to R3, R3's table looked like this before summarization:
R3#show ip route rip
172.16.0.0/24 is subnetted, 4 subnets
R 172.16.8.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
R 172.16.9.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
R 172.16.10.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
R 172.16.11.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
By summarizing the routes and using the ip summary-address command, RIP advertises only the summary route to the downstream neighbor.
R2(config)#int ethernet0
R2(config-if)#ip summary-address rip 172.16.8.0 255.255.252.0
R3#clear ip route *
R3#show ip route rip
172.16.0.0/22 is subnetted, 1 subnets
R 172.16.8.0 [120/1] via 172.23.23.2, 00:01:24, Ethernet0
EIGRP works much the same way, except that the EIGRP AS number must be named in the ip summary-address command.
In the following example, R2 was advertising four separate routes to R3 via EIGRP 100: 100.0.0.0, 101.0.0.0, 102.0.0.0, and 103.0.0.0, all with an eight-bit mask. What summary route can be used here?
The summary is 100.0.0.0 252.0.0.0. To send that route to downstream routers, configure the following on R2:
R2(config)#interface ethernet0
R2(config-if)#ip summary-address eigrp 100 100.0.0.0 252.0.0.0
R3 will then have only one route in its EIGRP table - the summary route.
R3#show ip route eigrp
D 100.0.0.0/6 [90/2297856] via 172.23.23.2, 00:02:33, Ethernet0
By mastering basic binary skills and keeping in mind that RIP and EIGRP perform route summarization at the interface level, you're one step closer to passing your BSCI exam and earning your CCNP certification!
In the next part of this tutorial, we'll take a detailed look at the different methods OSPF uses for route summarization.
Showing posts with label summary. Show all posts
Showing posts with label summary. Show all posts
Thursday, December 25, 2008
Cisco CCNP / BSCI Exam Tutorial: Route Summarization
Preparing to pass the BSCI exam and earn your Cisco CCNP? Route summarization is just one of the many skills you'll have to master in order to earn your CCNP. Whether it's RIP version 2, OSPF, or EIGRP, the BSCI exam will demand that you can flawlessly configure route summarization.
Route summarization isn't just important for the BSCI exam. It's a valuable skill to have in the real world as well. Correctly summarizing routes can lead to smaller routing tables that are still able to route packets accurately - what I like to call "concise and complete" routing tables.
The first skill you've got to have in order to work with route summarization is binary math more specifically, you must be able to take multiple routes and come up with both a summary route and mask to advertise to downstream routers. Given the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16, could you quickly come up with both the summary address and mask? All you need to do is break the four network numbers down into binary strings. We know the last two octets will all convert to the binary string 00000000, so in this article we'll only illustrate how to convert the first and second octet from decimal to binary.
100 16 = 01100100 00010000
100 17 = 01100100 00010001
100 18 = 01100100 00010010
100 19 = 01100100 00010011
To come up with the summary route, just work from left to right and draw a line where the four networks no longer have a bit in common. For these four networks, that point comes between the 14th and 15th bits. This leaves us with this string: 01100100 000100xx. All you need to do is convert that string back to decimal, which gives us 100 for the first octet and 16 for the second. (The two x values are bits on the right side of the line, which aren't used in calculating the summary route.) Since we know that zero is the value for the last two octets, the resulting summary network number is 100.16.0.0.
But we're not done! We now have to come up with the summary mask to advertise along with the summary route. To arrive at the summary route, write out a mask in binary with a "1" for every bit to the left of the line we drew previously, and a "0" for every bit to the right. That gives us the following string:
11111111 11111100 00000000 00000000
Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0. The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.
For the BSCI exam, emphasis is put on knowing how to advertise these summary routes in RIPv2, EIGRP, and OSPF. For RIP v2 and EIGRP, route summarization happens at the interface level - it's not configured under the protocol. On the interface that should advertise the summary route, use the command "ip summary-address". Here are examples of how the above summary route would be configured on ethernet0 in both RIPv2 and EIGRP.
R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0
R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0
The main difference between the two is that the EIGRP command must specify the AS number - that's what the "100" is in the middle of the EIGRP command. Since RIPv2 does not use AS numbers, there's no additional value needed in the configuration.
For OSPF, the commands differ. If you're configuring inter-area route summarization, use the "area range" command. The number following "area" is the area containing the routes being summarized, not the area receiving the summary.
R1(config)#router ospf 1
R1(config-router)#area 1 range 100.16.0.0 255.252.0.0
If you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR.
R1(config)#router ospf 1
R1(config-router)#summary-address 100.16.0.0 255.252.0.0
I speak from experience when I tell you that practice makes perfect on the BSCI exam, especially with binary and summarization questions. The great thing about these questions is that there are no grey areas with these questions - you either know how to do it or you don't. And with practice and an eye for detail, you can master these skills, pass the exam, and become a CCNP. Here's to your success on these tough Cisco certification exams!
Route summarization isn't just important for the BSCI exam. It's a valuable skill to have in the real world as well. Correctly summarizing routes can lead to smaller routing tables that are still able to route packets accurately - what I like to call "concise and complete" routing tables.
The first skill you've got to have in order to work with route summarization is binary math more specifically, you must be able to take multiple routes and come up with both a summary route and mask to advertise to downstream routers. Given the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16, could you quickly come up with both the summary address and mask? All you need to do is break the four network numbers down into binary strings. We know the last two octets will all convert to the binary string 00000000, so in this article we'll only illustrate how to convert the first and second octet from decimal to binary.
100 16 = 01100100 00010000
100 17 = 01100100 00010001
100 18 = 01100100 00010010
100 19 = 01100100 00010011
To come up with the summary route, just work from left to right and draw a line where the four networks no longer have a bit in common. For these four networks, that point comes between the 14th and 15th bits. This leaves us with this string: 01100100 000100xx. All you need to do is convert that string back to decimal, which gives us 100 for the first octet and 16 for the second. (The two x values are bits on the right side of the line, which aren't used in calculating the summary route.) Since we know that zero is the value for the last two octets, the resulting summary network number is 100.16.0.0.
But we're not done! We now have to come up with the summary mask to advertise along with the summary route. To arrive at the summary route, write out a mask in binary with a "1" for every bit to the left of the line we drew previously, and a "0" for every bit to the right. That gives us the following string:
11111111 11111100 00000000 00000000
Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0. The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.
For the BSCI exam, emphasis is put on knowing how to advertise these summary routes in RIPv2, EIGRP, and OSPF. For RIP v2 and EIGRP, route summarization happens at the interface level - it's not configured under the protocol. On the interface that should advertise the summary route, use the command "ip summary-address". Here are examples of how the above summary route would be configured on ethernet0 in both RIPv2 and EIGRP.
R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0
R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0
The main difference between the two is that the EIGRP command must specify the AS number - that's what the "100" is in the middle of the EIGRP command. Since RIPv2 does not use AS numbers, there's no additional value needed in the configuration.
For OSPF, the commands differ. If you're configuring inter-area route summarization, use the "area range" command. The number following "area" is the area containing the routes being summarized, not the area receiving the summary.
R1(config)#router ospf 1
R1(config-router)#area 1 range 100.16.0.0 255.252.0.0
If you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR.
R1(config)#router ospf 1
R1(config-router)#summary-address 100.16.0.0 255.252.0.0
I speak from experience when I tell you that practice makes perfect on the BSCI exam, especially with binary and summarization questions. The great thing about these questions is that there are no grey areas with these questions - you either know how to do it or you don't. And with practice and an eye for detail, you can master these skills, pass the exam, and become a CCNP. Here's to your success on these tough Cisco certification exams!
Cisco CCNP / BSCI Exam Tutorial: Using The OSPF Command “Area Range”
Your BSCI and CCNP exam success depends on knowing the details, and one such detail is knowing the proper way to summarize routes in OSPF. Route summarization is not just a test of your binary conversion abilities, but knowing where and when to summarize routes. It will not surprise any CCNA or CCNP certification candidate that OSPF gives us the most options for route summarization, and therefore more details to know!
OSPF offers us two options for route summarization configurations. In a previous tutorial, we looked at the "summary-address" command, and today we'll look at the proper use of the "area range" command.
The "area range" command should be used on an Area Border Router (ABR) to summarize routes being advertised from one OSPF area to another. In this tutorial, R1 is acting as an ABR, with interfaces in both Area 0 and Area 1. Four loopbacks have been placed into R1's Area 1.
R1(config)#router ospf 1
R1(config-router)#network 12.0.0.0 0.255.255.255 a 1
R1(config-router)#network 13.0.0.0 0.255.255.255 a 1
R1(config-router)#network 14.0.0.0 0.255.255.255 a 1
R1(config-router)#network 15.0.0.0 0.255.255.255 a 1
The routing table of an OSPF neighbor, R2, shows all four routes.
R2#show ip route ospf
12.0.0.0/32 is subnetted, 1 subnets
O IA 12.12.12.12 [110/65] via 172.12.123.1, 00:18:52, Serial0
13.0.0.0/32 is subnetted, 1 subnets
O IA 13.13.13.13 [110/65] via 172.12.123.1, 00:18:42, Serial0
14.0.0.0/32 is subnetted, 1 subnets
O IA 14.14.14.14 [110/65] via 172.12.123.1, 00:18:32, Serial0
15.0.0.0/32 is subnetted, 1 subnets
O IA 15.15.15.15 [110/65] via 172.12.123.1, 00:18:32, Serial0
To keep the routing tables of downstream routers smaller but still have the desired IP connectivity, we can use the area range command on R1 to summarize these four routes. The key to keep in mind with the area range command is that the area number given in the command is the area containing the destinations, NOT the area that will receive the summary route.
R1(config)#router ospf 1
R1(config-router)#area 1 range 12.0.0.0 252.0.0.0
R2 now shows a single summary route that can be used to reach all four remote networks.
R2#show ip route ospf
O IA 12.0.0.0/6 [110/65] via 172.12.123.1, 00:00:21, Serial0
Interestingly enough, there's now an additional route in R1's routing table.
R1#show ip route ospf
O 12.0.0.0/6 is a summary, 00:07:53, Null0
When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table of the router performing the summarization. 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, and packets that do not match one of the summarized routes but do match the summary route will be dropped.
OSPF offers us two options for route summarization configurations. In a previous tutorial, we looked at the "summary-address" command, and today we'll look at the proper use of the "area range" command.
The "area range" command should be used on an Area Border Router (ABR) to summarize routes being advertised from one OSPF area to another. In this tutorial, R1 is acting as an ABR, with interfaces in both Area 0 and Area 1. Four loopbacks have been placed into R1's Area 1.
R1(config)#router ospf 1
R1(config-router)#network 12.0.0.0 0.255.255.255 a 1
R1(config-router)#network 13.0.0.0 0.255.255.255 a 1
R1(config-router)#network 14.0.0.0 0.255.255.255 a 1
R1(config-router)#network 15.0.0.0 0.255.255.255 a 1
The routing table of an OSPF neighbor, R2, shows all four routes.
R2#show ip route ospf
12.0.0.0/32 is subnetted, 1 subnets
O IA 12.12.12.12 [110/65] via 172.12.123.1, 00:18:52, Serial0
13.0.0.0/32 is subnetted, 1 subnets
O IA 13.13.13.13 [110/65] via 172.12.123.1, 00:18:42, Serial0
14.0.0.0/32 is subnetted, 1 subnets
O IA 14.14.14.14 [110/65] via 172.12.123.1, 00:18:32, Serial0
15.0.0.0/32 is subnetted, 1 subnets
O IA 15.15.15.15 [110/65] via 172.12.123.1, 00:18:32, Serial0
To keep the routing tables of downstream routers smaller but still have the desired IP connectivity, we can use the area range command on R1 to summarize these four routes. The key to keep in mind with the area range command is that the area number given in the command is the area containing the destinations, NOT the area that will receive the summary route.
R1(config)#router ospf 1
R1(config-router)#area 1 range 12.0.0.0 252.0.0.0
R2 now shows a single summary route that can be used to reach all four remote networks.
R2#show ip route ospf
O IA 12.0.0.0/6 [110/65] via 172.12.123.1, 00:00:21, Serial0
Interestingly enough, there's now an additional route in R1's routing table.
R1#show ip route ospf
O 12.0.0.0/6 is a summary, 00:07:53, Null0
When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table of the router performing the summarization. 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, and packets that do not match one of the summarized routes but do match the summary route will be dropped.
Cisco CCNP / BSCI Exam Tutorial: Using OSPF's "Summary-Address" Command
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.
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.
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!
Cisco CCNA Exam Tutorial: Route Summarization
Preparing to pass the CCNA exam and earn this important Cisco certification? Route summarization is just one of the many skills you'll have to master in order to earn your CCNA. Whether it's RIP version 2, OSPF, or EIGRP, the CCNA exam will demand that you can flawlessly configure route summarization.
Route summarization isn't just important for the CCNA exam. It's a valuable skill to have in the real world as well. Correctly summarizing routes can lead to smaller routing tables that are still able to route packets accurately - what I like to call "concise and complete" routing tables.
The first skill you've got to have in order to work with route summarization is binary math; more specifically, you must be able to take multiple routes and come up with both a summary route and mask to advertise to downstream routers. Given the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16, could you quickly come up with both the summary address and mask? All you need to do is break the four network numbers down into binary strings. We know the last two octets will all convert to the binary string 00000000, so in this article we'll only illustrate how to convert the first and second octet from decimal to binary.
100 16 = 01100100 00010000
100 17 = 01100100 00010001
100 18 = 01100100 00010010
100 19 = 01100100 00010011
To come up with the summary route, just work from left to right and draw a line where the four networks no longer have a bit in common. For these four networks, that point comes between the 14th and 15th bits. This leaves us with this string: 01100100 000100xx. All you need to do is convert that string back to decimal, which gives us 100 for the first octet and 16 for the second. (The two x values are bits on the right side of the line, which aren't used in calculating the summary route.) Since we know that zero is the value for the last two octets, the resulting summary network number is 100.16.0.0.
But we're not done! We now have to come up with the summary mask to advertise along with the summary route. To arrive at the summary route, write out a mask in binary with a "1" for every bit to the left of the line we drew previously, and a "0" for every bit to the right. That gives us the following string:
11111111 11111100 00000000 00000000
Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0. The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.
For the CCNA exam, emphasis is put on knowing how to advertise these summary routes in RIPv2 and EIGRP. For both of these protocols, route summarization happens at the interface level - it's not configured under the protocol. On the interface that should advertise the summary route, use the command "ip summary-address". Here are examples of how the above summary route would be configured on ethernet0 in both RIPv2 and EIGRP.
R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0
R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0
The main difference between the two is that the EIGRP command must specify the AS number - that's what the "100" is in the middle of the EIGRP command. Since RIPv2 does not use AS numbers, there's no additional value needed in the configuration.
For OSPF, the commands differ. If you're configuring inter-area route summarization, use the "area range" command; if you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR. Neither of these are interface-level commands.
I speak from experience when I tell you that practice makes perfect on the CCNA exam, especially with binary and summarization questions. The great thing about these questions is that there are no grey areas with these questions - you either know how to do it or you don't. And with practice and an eye for detail, you can master these skills, pass the exam, and become a CCNA. Here's to your success!
Route summarization isn't just important for the CCNA exam. It's a valuable skill to have in the real world as well. Correctly summarizing routes can lead to smaller routing tables that are still able to route packets accurately - what I like to call "concise and complete" routing tables.
The first skill you've got to have in order to work with route summarization is binary math; more specifically, you must be able to take multiple routes and come up with both a summary route and mask to advertise to downstream routers. Given the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16, could you quickly come up with both the summary address and mask? All you need to do is break the four network numbers down into binary strings. We know the last two octets will all convert to the binary string 00000000, so in this article we'll only illustrate how to convert the first and second octet from decimal to binary.
100 16 = 01100100 00010000
100 17 = 01100100 00010001
100 18 = 01100100 00010010
100 19 = 01100100 00010011
To come up with the summary route, just work from left to right and draw a line where the four networks no longer have a bit in common. For these four networks, that point comes between the 14th and 15th bits. This leaves us with this string: 01100100 000100xx. All you need to do is convert that string back to decimal, which gives us 100 for the first octet and 16 for the second. (The two x values are bits on the right side of the line, which aren't used in calculating the summary route.) Since we know that zero is the value for the last two octets, the resulting summary network number is 100.16.0.0.
But we're not done! We now have to come up with the summary mask to advertise along with the summary route. To arrive at the summary route, write out a mask in binary with a "1" for every bit to the left of the line we drew previously, and a "0" for every bit to the right. That gives us the following string:
11111111 11111100 00000000 00000000
Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0. The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.
For the CCNA exam, emphasis is put on knowing how to advertise these summary routes in RIPv2 and EIGRP. For both of these protocols, route summarization happens at the interface level - it's not configured under the protocol. On the interface that should advertise the summary route, use the command "ip summary-address". Here are examples of how the above summary route would be configured on ethernet0 in both RIPv2 and EIGRP.
R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0
R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0
The main difference between the two is that the EIGRP command must specify the AS number - that's what the "100" is in the middle of the EIGRP command. Since RIPv2 does not use AS numbers, there's no additional value needed in the configuration.
For OSPF, the commands differ. If you're configuring inter-area route summarization, use the "area range" command; if you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR. Neither of these are interface-level commands.
I speak from experience when I tell you that practice makes perfect on the CCNA exam, especially with binary and summarization questions. The great thing about these questions is that there are no grey areas with these questions - you either know how to do it or you don't. And with practice and an eye for detail, you can master these skills, pass the exam, and become a CCNA. Here's to your success!
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