Basic IS-IS architecture, wierd behaviour

Unanswered Question
Mar 14th, 2008

Hi all

I have a very basic IS-IS lab as shown in attached figure and I have some easy questions.

1) I am implementing “Example S-12” in http://www.ciscopress.com/articles/article.asp?p=31319&seqNum=4 in my attached lab diagram. IS-IS is configured in all routers, and all can communicate atm no problem. But something looks weird.

If I issue the “isis circuit-type level-1” in s1/0 interface of P1R1 as suggested in article, the routing table in P1R3 becomes as following

P1R3#sh ip ro

Codes: C - connected, S - static, 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

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default, U - per-user static route

o - ODR, P - periodic downloaded static route

Gateway of last resort is 192.168.1.49 to network 0.0.0.0

192.168.1.0/28 is subnetted, 4 subnets

i L1 192.168.1.32 [115/20] via 192.168.1.49, FastEthernet0/0

C 192.168.1.48 is directly connected, FastEthernet0/0

i L1 192.168.1.0 [115/30] via 192.168.1.49, FastEthernet0/0

i L1 192.168.1.16 [115/20] via 192.168.1.49, FastEthernet0/0

i*L1 0.0.0.0/0 [115/20] via 192.168.1.49, FastEthernet0/0

And the following isis neighborhood output from P1R2

P1R2#sh isis neighbors

System Id Type Interface IP Address State Holdtime Circuit Id

P1R1 L1 Se1/0 192.168.1.33 UP 28 00

P1R3 L1 Fa0/0 192.168.1.50 UP 7 P1R3.01

P1R3 L2 Fa0/0 192.168.1.50 UP 9 P1R3.01

If I remove it by "no isis circuit-type level-1" from s1/0 in P1R1, route table in P1R3 changes as following

P1R3#sh ip ro

Codes: C - connected, S - static, 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

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default, U - per-user static route

o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 1 subnets

i L2 172.16.1.0 [115/40] via 192.168.1.49, FastEthernet0/0

10.0.0.0/24 is subnetted, 1 subnets

i L2 10.100.100.0 [115/30] via 192.168.1.49, FastEthernet0/0

192.168.1.0/28 is subnetted, 4 subnets

i L1 192.168.1.32 [115/20] via 192.168.1.49, FastEthernet0/0

C 192.168.1.48 is directly connected, FastEthernet0/0

i L1 192.168.1.0 [115/30] via 192.168.1.49, FastEthernet0/0

i L1 192.168.1.16 [115/20] via 192.168.1.49, FastEthernet0/0

192.168.2.0/28 is subnetted, 4 subnets

i L2 192.168.2.32 [115/40] via 192.168.1.49, FastEthernet0/0

i L2 192.168.2.48 [115/50] via 192.168.1.49, FastEthernet0/0

i L2 192.168.2.0 [115/40] via 192.168.1.49, FastEthernet0/0

i L2 192.168.2.16 [115/50] via 192.168.1.49, FastEthernet0/0

And isis neighborhood in P1R2

P1R2#sh isis neighbors

System Id Type Interface IP Address State Holdtime Circuit Id

P1R1 L1L2 Se1/0 192.168.1.33 UP 23 00

P1R3 L1 Fa0/0 192.168.1.50 UP 6 P1R3.01

P1R3 L2 Fa0/0 192.168.1.50 UP 7 P1R3.01

Why does P1R1 advertise a default static route when the circuit-type is issued? There may be a case in which default route is somewhere else so I dont want it to advertise default route.

"ia - IS-IS inter area" i dont see any ia entry in route table? I just see L1 and L2

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husycisco Fri, 03/14/2008 - 16:08

2)In P1R1, "summary-address 192.168.1.0 255.255.255.0" is issued in "router isis" . P2R1 sees the summarized address in its route table when "isis circuit-type level-1" is issued in s1/0 of P1R1. If I remove it, P2R1 will see both summarized and not summarized addresses

3)Either circuit-type command is issued or not, P2R2 and P2R3 does see the summarized address created by P2R1, which they shouldnt infact. They should only see the summarized address advertised by P1R1

Thanks for your comments

Istvan_Rabai Sat, 03/15/2008 - 06:12

Hi Huseyin,

The original config for P1R1 s1/0 is L1/L2 circuit type.

This is why P1R1 will form a L1/L2 neighbor relationship with P1R2 and pass L2 routes to P1R2. P1R2 further advertises the L2 routes to P1R3.

You can observe the L2 routes in P1R3 routing table, coming from P2R1, P2R2 and P2R3.

When you issue the "isis circuit-type level-1" command on s1/0 of P1R1, the L2 neighbor relationship with P1R2 is broken.

Their neighbor relationship will be L1 only.

P1R1 will not pass L2 routes to P1R2.

P1R1 starts advertising a default route into its L1 area 01.0001. (to P1R2). So P1R2 and P1R3 both should have a default route in their routing tables.

P1R1 will continue to have the L2 neighbor relationship with P2R1, as they form the backbone area.

As soon as you issue the "no isis circuit-type level-1" command, the setting becomes the default L1/L2 circuit type for s1/0 and the L1/L2 neighbor relationship will be formed betwen P1R1 and P1R2.

So the L2 routes will appear again in P1R2 and P1R3 routing tables.

This is the normal behavior of is-is.

Cheers:

Istvan

husycisco Sat, 03/15/2008 - 07:37

Hi Istvan

Thanks for your valuable time and explaination which makes sense. I once thought this was not the normal behaviour since I have not found any documentation about this issue except the one I posted.

But if I issue "no isis circuit-type level-1" as it should be, unlike the article, also you and I agree that this is the original config for P1R1 s1/0, subnets of P1 routers start to appear in route tables of P2 routers although a summarry-address is configured in P1R1. Both summary address and not summarized addresses start appearing in route tables of P2 routers

Another wierd issue is, you know P1R1 creates a Null0 interface for summarization. As soon as I issue "no isis circuit-type level-1" summary address gets advertised to P1R2 and P1R3. This shouldnt be happening.

Do you have any thoughts about this?

Istvan_Rabai Sat, 03/15/2008 - 12:32

Hi Huseyin,

Basically, the configuration that you created is a little bit weird, this is why you have weird results.

IS-IS works well when you have a hierarchical design: you have a backbone area with L2 or L1/L2 routers and you have stub areas with L1 routers.

If you issue the "no isis circuit-type level-1" command, then the whole network becomes ONE L2 backbone, which is weird.

For a scalable working design, areas 01.0001 and 01.0002 should be L1 (stub) areas, and the P1R1 and P2R1 should be the backbone L1/L2 routers, injecting default routes into their L1 areas.

So the best is to issue the "isis circuit-type level-1" on both P1R1 and P2R1 s1/0 interfaces.

Cheers:

Istvan

husycisco Wed, 03/19/2008 - 16:06

"Basically, the configuration that you created is a little bit weird, this is why you have weird results"

I created the simplest design ever possible. You can remove P1R3 and P2R3 if you like, questions are still the same. And I applied the documentation from cisco

"So the best is to issue the "isis circuit-type level-1" on both P1R1 and P2R1 s1/0 interfaces."

Is the best or only since summarization does not work?

Do you have another simple lab (a documentation, link whatever) for me to apply?

Istvan_Rabai Wed, 03/19/2008 - 23:47

Hi Huseyin,

I meant the software configuration a little bit weird because the whole network would be one L2 backbone. This is not usual.

Let P1R2 and P1R3 be L1 routers, and P2R2 and P2R3 L1 routers.

Let P1R1 and P2R1 be L1/L2 routers.

Try to use the "summary-address" command with the "level-2" option on P1R1 and P2R1.

This will summarize their selected routes into the L2 backbone.

This will be a regular hierarchical configuration and it should work.

Cheers:

Istvan

husycisco Tue, 03/25/2008 - 09:09

Hi Istvan

Thanks for your time, I will try this further and let you know.

Regards

Istvan_Rabai Tue, 03/25/2008 - 13:20

You're welcome!

Of course I'm curious about your progress with this. Looking forward to hearing from you.

And thank you for the ratings!

Cheers:

Istvan

ruwhite Fri, 03/28/2008 - 05:20

I may be missing something, but this just looks like normal IS-IS operation to me (?). In IS-IS, all L1 flooding domains are similar to OSPF totally not-so-stubby areas by default, so the L1/L2 border sets the attached bit, which translates into a default route within the entire L1 flooding domain, and L2 routes are not injected into the L1 flooding domain, either. L1's are translated into the L1 domain, much like an OSPF type 3, but not the other way around.

It seems to me that when you change the link type, you are allowing L2 to run on the link, which means the L2 domain extends to the router you're looking at, which means it gets all the routers, and there's no default injected through the attached bit.

:-)

Russ

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