IS-IS Adjacency question

rakem

In the cisco press CCNP BSCI Office Exam Cert Guide 4th edition, there is a question about IS-IS that says:

For an IS-IS adjacency to be formed and maintained, both interfaces must agree on which of the following:
a) If Level 1 both must be in the same area
b) System ID must be unique
c) Both must be configured at the same level of routing (1 or 2)
d) hello timers

Answers given are A and B.

I would of though that C and D should be correct as well, and even in the book, on page 265, it actually says that hello timers must match, and that both routers must support the same level of routing....

ok maybe not C since the the question says that they must be configured in the same area, and the book says the must support the same area.... but i still think D should still be correct

cristi.grigore

This is a bit strange: I tried this out between two routers and it seems the isis hello timers don't have to match.

R1

---

R2

Here is the info from R1:

R1#sh ip int b
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES NVRAM  administratively down down
FastEthernet1/0            192.168.1.1     YES NVRAM  up                    up
FastEthernet2/0            192.168.2.1     YES NVRAM  up                    up
Loopback1                  200.1.1.1       YES NVRAM  up                    up
R1#sh run int f 1/0
Building configuration...

Current configuration : 127 bytes
!
interface FastEthernet1/0
 ip address 192.168.1.1 255.255.255.0
 ip router isis 1
 duplex half
 isis hello-interval 100
end

R1#sh isis ne
R1#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R2             L1   Fa1/0       192.168.1.2     UP    2        R2.01
R2             L2   Fa1/0       192.168.1.2     UP    2        R2.01
R1#

And here is R2:

R2#sh ip int b
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES NVRAM  administratively down down
FastEthernet1/0            192.168.1.2     YES NVRAM  up                    up
FastEthernet2/0            192.168.3.1     YES NVRAM  up                    up
Loopback1                  200.2.2.2       YES NVRAM  up                    up
R2#sh run int f 1/0
Building configuration...

Current configuration : 125 bytes
!
interface FastEthernet1/0
 ip address 192.168.1.2 255.255.255.0
 ip router isis 1
 duplex half
 isis hello-interval 3
end

R2#sh isis ne
R2#sh isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R1             L1   Fa1/0       192.168.1.1     UP    229      R2.01
R1             L2   Fa1/0       192.168.1.1     UP    249      R2.01
R2#

I also took a capture of the isis hello packets and inside there is only the holdtime field, not the hello field. This leads me to guess that R1 - which has a 100 hello interval - will only advertise the hold timer (three times the hello timer) to R2. This means that the timers don;t have to match. The one thing I don't get is why R1 sees a hold timer of 3 for R2 (which has the hello interval set also to three) - it should be 9.

Cucumber

Indeed, it is not necessary for two ISIS routers to have the same timers in order to become
adjacent.

For instance, by default, routers use a 10 second Hello time, but some specific routers, the designated ones for multiaccess networks use a 3.3 sec hello time. So, on multiaccess networks, the DIS use a 3.3 Hello time, whereas all other routers use a 10 second Hello timer, and still everyone become adjacent to everyone on these networks.

Therefore D is wrong.

Option A is correct, for Level-1 routers, they must belong to the same area.

Regarding C, it sounds correct to me. It just does not seems to fit for routers that are level 1-2.

Answer B is a required condition for IS-IS to work ( the whole IS-IS protocol relies on the fact all the routers have an unique ID), so its odd this is specifically mentioned for adjacencies.

Mr.Ping

Maybe I can help shed some light on C.

Two L1-only routers form an L1 adjacency only if their AIDs match.

Two L2-only routers form an L2 adjacency, even if their AIDs are different.

An L1-only router forms an L1 adjacency with an L1/L2 router only if their AIDs match.

An L2-only router forms an L2 adjacency with an L1/L2 router even if their AIDs are different.

Two L1/L2 routers form both L1 and L2 adjacencies if their AIDs match.

Two L1/L2 routers form only an L2 adjacency if their AIDs do not match.



Unlike OSPF the timers don't need to match for ISIS.

courtesy of:

CCIE Professional Development Routing TCP/IP, Volume I, Second Edition
By Jeff Doyle - CCIE No. 1919, Jennifer Carroll - CCIE No. 1402

rakem

ok well thanks for the help guys... i just the book must be wrong cause it clearly says that hello timers must match...