CCNP Tutorial For The BSCI Exam:

ISIS And Manual Route Summarization

(Additional Summarization Info Added Mar. 12)

Chris Bryant, CCIE #12933

Vital Reading And Watching For All Cisco Certification Candidates:

The July 2010 CCNP Changes (And How To Beat Them!)

All right, Bulldogs, here we go with another ISIS tutorial to help you beat the July 31, 2010 BSCI exam deadline - and this is a big one!

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Be sure to visit the BSCI Exam Tutorial Page for plenty of additional CCNP tutorials - and this ISIS tutorial picks up where another left off.

Here's the network we're working with, and keep in mind that our Frame Relay network in this network is full-mesh, not the typical hub-and-spoke we've worked with in most of our CCNA and BSCI labs.

ISIS allows manual route summarization, but there are some restrictions. (Aren't there always?)

ISIS internal route summarization cannot take place inside an area, so it cannot be performed on an L1 router. 

Internal routes can be summarized between areas, though, turning multiple L1 routes into a single L2 route.  Therefore, an L1/L2 router is where route summarization should take place.

In the above network, let's add the networks 148.0.0.0 /8, 149.1.1.0 /8, 150.0.0.0 /8, and 151.0.0.0 /8 to R7 and then view the routing tables on R5 and R4, the L2 routers.

We'll use loopbacks on R7 to add those networks - and don't forget to add ip router isis to each loopback!

R7(config)#int loopback 148
R7(config-if)#ip address 148.7.7.7 255.0.0.0
R7(config-if)#ip router isis  

R7(config-if)#int loopback 149
R7(config-if)#ip address 149.7.7.7 255.0.0.0
R7(config-if)#ip router isis

R7(config-if)#int loopback 150
R7(config-if)#ip address 150.7.7.7 255.0.0.0
R7(config-if)#ip router isis

R7(config-if)#int loopback 151
R7(config-if)#ip address 151.7.7.7 255.0.0.0
R7(config-if)#ip router isis

Now let's check out R5's ISIS routing table:

R5#show ip route isis
     46.0.0.0/24 is subnetted, 1 subnets
i L2    46.1.1.0 [115/20] via 172.12.123.4, Serial0/1
i L1 149.0.0.0/8 [115/20] via 51.1.1.7, FastEthernet0/0
i L1 148.0.0.0/8 [115/20] via 51.1.1.7, FastEthernet0/0
i L1 151.0.0.0/8 [115/20] via 51.1.1.7, FastEthernet0/0
i L1 150.0.0.0/8 [115/20] via 51.1.1.7, FastEthernet0/0

R5 has four internal routes that can be summarized (the bottom four). You might think the summarization should take place on R7, but this summarization needs to be performed on an L1/L2 router. 

ISIS route summarization should always take place on the area border router, which in ISIS terminology is the L1/L2 router.

Now without scrolling down, perform that route summarization, and then we'll apply it to the network!

Done? Then scroll down... after taking just a moment to read this important message!

Earn Your CCNP With The Personal Guarantee Of Chris Bryant, CCIE #12933:

“I GUARANTEE You'll Pass The Current CCNP Exams - BSCI, ONT, ISCW, and BCMSN - Before The July 31, 2010 Cutoff Date With My CCNP Study Packages ...

... And If You Don't Pass FOR ANY REASON Before That Time, I'll Give You a 100% Free CCNP Study Package Download For The New Exam Track!"

You Also Get A FREE CCNA Security Study Package, Valued at $67, With The Purchase Of Any CCNP Study Package!

Let's pick back up with our ISIS tutorial!

The summary network to be advertised is 148.0.0.0 252.0.0.0.  Let's use the summary-address command on R5 to perform that summarization, and then we'll go to R4 to verify our config!

R5(config)#router isis
R5(config-router)#summary-address 148.0.0.0 252.0.0.0

R4#show ip route isis
     51.0.0.0/24 is subnetted, 1 subnets
i L2    51.1.1.0 [115/20] via 172.12.123.5, Serial0/0
i L2 148.0.0.0/6 [115/30] via 172.12.123.5, Serial0/0

The other L1/L2 router, R4, shows the summary route instead of the more-specific routes. 

More importantly, we need to be sure that R8 can ping all of the summarized addresses. Can it?

R8#ping 148.7.7.7

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 148.7.7.7, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 68/285/685 ms

R8#ping 149.7.7.7

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 149.7.7.7, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/65/69 ms

R8#ping 150.7.7.7

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.7.7.7, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/112/249 ms

R8#ping 151.7.7.7

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 151.7.7.7, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 64/137/420 ms

You can see where the use of L1 routers and route summarization has greatly shrunk the size of the routing tables. 

R8 uses a single default route to reach five different networks contained in another area, and R4 uses a single summary route to reach four separate networks in another area.

This might surprise you, though - there's a new route in R5's ISIS table as well.

R5#show ip route isis
i su 148.0.0.0/6 [115/10] via 0.0.0.0, Null0

The code  i su indicates an ISIS summary route.  This null route is placed into the routing table automatically when a summary ISIS route is created. 

This is to prevent routing loops from forming in case packets are received that do not match the more-specific routes on this router, but that do match the summary route. 

When using ISIS summary routes, we need to be cautious when there are multiple L1/L2 routers connecting two areas, as in the following example.

(This is also fertile ground for potential routing loops, but that's a subject for another tutorial.)

If one of our L1/L2 routers in this scenario is advertising an ISIS summary, the other L1/L2s should do so as well.

If one of them is advertising a summary while the other advertises the specific routes, all traffic would take the path through the router advertising the specific routes due to the longest match rule. 

In the above example, both R5 and R6 must advertise the summary route.  If R5 advertised the more-specific routes instead, that router would end up being the transit router for all traffic leaving Area 49.0002 that is destined for any of the summarized routes - and that's a waste of bandwidth and ends up putting a heavy load on R5.

For plenty of additional BSCI and ISIS tutorials, visit our BSCI Tutorial Page and CCNP Practice Exam pages!

Earn Your CCNP With The Personal Guarantee Of Chris Bryant, CCIE #12933:

“I GUARANTEE You'll Pass The Current CCNP Exams - BSCI, ONT, ISCW, and BCMSN - Before The July 31, 2010 Cutoff Date With My CCNP Study Packages ...

... And If You Don't Pass FOR ANY REASON Before That Time, I'll Give You a 100% Free CCNP Study Package Download For The New Exam Track!"

You Also Get A FREE CCNA Security Study Package, Valued at $67, With The Purchase Of Any CCNP Study Package!

Chris Bryant

CCIE #12999

"The Computer Certification Bulldog"

chris@thebryantadvantage.com