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!
Showing posts with label conversion. Show all posts
Showing posts with label conversion. Show all posts
Thursday, December 25, 2008
Monday, December 22, 2008
CCNP Certification / BCMSN Exam Tutorial: HSRP MAC Addresses And Timers
To earn your CCNP certification and pass the BCMSN exam, you've got to know what HSRP does and the many configurable options. While the operation of HSRP is quite simple (and covered in a previous tutorial), you also need to know how HSRP arrives at the MAC address for the virtual router - as well as how to configure a new MAC for this virtual router. This puts us in the unusual position of creating a physical address for a router that doesn't exist!
The output of show standby for a two-router HSRP configuration is shown below.
R2#show standby
Ethernet0 - Group 5
Local state is Standby, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 0.776
Virtual IP address is 172.12.23.10 configured
Active router is 172.12.23.3, priority 100 expires in 9.568
Standby router is local
1 state changes, last state change 00:00:22
R3#show standby
Ethernet0 - Group 5
Local state is Active, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 2.592
Virtual IP address is 172.12.23.10 configured
Active router is local
Standby router is 172.12.23.2 expires in 8.020
Virtual mac address is 0000.0c07.ac05
2 state changes, last state change 00:02:08
R3 is in Active state, while R2 is in Standby. The hosts are using the 172.12.123.10 address as their gateway, but R3 is actually handling the workload. R2 will take over if R3 becomes unavailable.
An IP address was assigned to the virtual router during the HSRP configuration process, but not a MAC address. However, there is a MAC address under the show standby output on R3, the active router. How did the HSRP process arrive at a MAC of 00-00-0c-07-ac-05?
Well, most of the work is already done before the configuration is even begun. The MAC address 00-00-0c-07-ac-xx is reserved for HSRP, and xx is the group number in hexadecimal. That's a good skill to have for the exam, so make sure you're comfortable with hex conversions. The group number is 5, which is expressed as 05 with a two-bit hex character. If the group number had been 17, we'd see 11 at the end of the MAC address - one unit of 16, one unit of 1.
The output of the show standby command also tells us that the HSRP speakers are sending Hellos every 3 seconds, with a 10-second holdtime. These values can be changed with the standby command, but HSRP speakers in the same group should have the same timers. You can even tie down the hello time to the millisecond, but it's doubtful you'll ever need to do that.
R3(config-if)#standby 5 timers ?
<1-254> Hello interval in seconds
msec Specify hello interval in milliseconds
R3(config-if)#standby 5 timers 4 ?
<5-255> Hold time in seconds
R3(config-if)#standby 5 timers 4 12
Another important HSRP skill is knowing how to change the Active router assignment. I'll show you how to do that, and how to configure HSRP interface tracking, in the next part of my CCNP / BCMSN exam tutorial!
The output of show standby for a two-router HSRP configuration is shown below.
R2#show standby
Ethernet0 - Group 5
Local state is Standby, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 0.776
Virtual IP address is 172.12.23.10 configured
Active router is 172.12.23.3, priority 100 expires in 9.568
Standby router is local
1 state changes, last state change 00:00:22
R3#show standby
Ethernet0 - Group 5
Local state is Active, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 2.592
Virtual IP address is 172.12.23.10 configured
Active router is local
Standby router is 172.12.23.2 expires in 8.020
Virtual mac address is 0000.0c07.ac05
2 state changes, last state change 00:02:08
R3 is in Active state, while R2 is in Standby. The hosts are using the 172.12.123.10 address as their gateway, but R3 is actually handling the workload. R2 will take over if R3 becomes unavailable.
An IP address was assigned to the virtual router during the HSRP configuration process, but not a MAC address. However, there is a MAC address under the show standby output on R3, the active router. How did the HSRP process arrive at a MAC of 00-00-0c-07-ac-05?
Well, most of the work is already done before the configuration is even begun. The MAC address 00-00-0c-07-ac-xx is reserved for HSRP, and xx is the group number in hexadecimal. That's a good skill to have for the exam, so make sure you're comfortable with hex conversions. The group number is 5, which is expressed as 05 with a two-bit hex character. If the group number had been 17, we'd see 11 at the end of the MAC address - one unit of 16, one unit of 1.
The output of the show standby command also tells us that the HSRP speakers are sending Hellos every 3 seconds, with a 10-second holdtime. These values can be changed with the standby command, but HSRP speakers in the same group should have the same timers. You can even tie down the hello time to the millisecond, but it's doubtful you'll ever need to do that.
R3(config-if)#standby 5 timers ?
<1-254> Hello interval in seconds
msec Specify hello interval in milliseconds
R3(config-if)#standby 5 timers 4 ?
<5-255> Hold time in seconds
R3(config-if)#standby 5 timers 4 12
Another important HSRP skill is knowing how to change the Active router assignment. I'll show you how to do that, and how to configure HSRP interface tracking, in the next part of my CCNP / BCMSN exam tutorial!
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