[c-nsp] ASR 1k, 3.1.0S MTU issues on PortChannel interfaces

[Pshem Kowalczyk]

Hi,

I'm currently trying to test the following scenario:

ASR1k ---- 4900M ----- ASR9k

the 1k runs 3.1.0S (or 15.0(1)S)
the 9k runs 3.9.1 (XR)

as the link between the 4900M and 9k is provided by a third party we
only have a MTU of 4400 there. There are 3 links between 1k and 4900M.
I'm struggling with getting the ISIS to come up, as I suspect due to
MTU on the PortChannel interface:

Port-channel1 is up, line protocol is up
  Hardware is GEChannel, address is 5475.d089.a4c0 (bia 5475.d089.a4c0)
  MTU 4400 bytes, BW 30000000 Kbit/sec, DLY 10 usec,
<-------------------------------------------
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  1., loopback not set
  Keepalive set (10 sec)
  ARP type: ARPA, ARP Timeout 04:00:00
    No. of active members in this channel: 3
        Member 0 : TenGigabitEthernet0/0/0 , Full-duplex, 10000Mb/s
        Member 1 : TenGigabitEthernet1/0/0 , Full-duplex, 10000Mb/s
        Member 2 : TenGigabitEthernet2/0/0 , Full-duplex, 10000Mb/s
    No. of PF_JUMBO supported members in this channel : 3

interface Port-channel1
 mtu 4400
 no ip address
end

Port-channel1.600 is up, line protocol is up
  Hardware is GEChannel, address is 5475.d089.a4c0 (bia 5475.d089.a4c0)
  Description: SPNZSKY01
  Internet address is 10.123.223.2/29
  MTU 1500 bytes, BW 30000000 Kbit/sec, DLY 10 usec,
<-------------------------------------------
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation 802.1Q Virtual LAN, Vlan ID  600.

interface Port-channel1.600
 description SPNZSKY01
 encapsulation dot1Q 600
 ip address 10.123.223.2 255.255.255.248
 ip mtu 4396
 ip router isis vfnz-core
 mpls ip
 isis network point-to-point
 isis authentication mode md5
 isis authentication key-chain VFNZ-KEY-CHAIN
 isis csnp-interval 10

I tried multiple things already to change that MTU, but it just
doesn't want to move (removing the config and re-applying it doesn't
work either)

when I debug isis adj I get the following:

*Nov 17 15:50:46.998: ISIS-Adj: Sending serial IIH on
Port-channel1.600, length 1496


when I look on the 9k I can see:
RP/0/RSP0/CPU0:Nov 18 11:57:19.616 : isis[285]: SEND P2P IIH (L2) on
TenGigE0/0/0/1.600: Holdtime 30s, Length 4379
RP/0/RSP0/CPU0:Nov 18 11:57:24.163 : isis[285]: RECV P2P IIH (L2) from
TenGigE0/0/0/1.600 SNPA 5475.d089.a4c0: System ID BGAUESD01, Holdtime
30, length 1496

I can not ping with anything bigger then 1500 from the 1K either.

The 4900M has all interfaces set to MTU of 9196 (max for the platform).

Any idea how to change that mtu?

kind regards
Pshem