IS-IS hello padding

[dsplittberger]

I'm working on a 7600 series router and noticed that when sending IS-IS padded hello's, the hello's do not actually get padded out to the CLNS MTU value, but actually get padded out to the MTU - 1.

So for example, if I just have the interface MTU set to say 9000 then doing a show clns interface would show the clns MTU at 8997, which is correct. But when debugging, I see that the router is sending 8996 PDU's. Now if I set the CLNS MTU to say 7000, debugging shows the router sending PDU's of 6999 size. Unless I'm misreading something, the hello's should be padded to the size of the MTU, so where is this 1 byte going, why pad to one less than the MTU?

[rsimoni]

Hi David,

nice catch!!!! 

the maximum size of isis hello packet over point-to-point link is purposely set to clns_mtu -1 becasue  we need extra bite to send over DS3 links (for reasons related to IOS code that I cannot comment here). This is done for historical reason and changing the behavior may cause issues with DS3 link.  So this is expected behavior.

By the way on broadcast interfaces the padded hello packet should be the entire clns_mtu value.

Lastly he behavior is common on all IOS platforms and not only related to 7600.

Hope this helps

Riccardo

[dsplittberger]

Much appreciated on the explanation Riccardo. It was driving me nuts trying to figure out why the extra byte was missing.

[rsimoni]

you are welcome

[yulliu]

ISIS for IPV6 between ASR9K and ASR1K

Need to set up the hello-padding disable on both sides. Otherwise, the ipv6 isis route can't be got by each other.

router isis area2
.....
 interface TenGigE0/2/0/0.6
  circuit-type level-2-only
  hello-padding disable
  address-family ipv6 unicast

[Harold Ritter]

This is required because the interface mtu is likely different on either side.

Regards,