[c-nsp] EoMPLS VC on Cisco 7609 is up, but with very small MTU (42 bytes)

[Saku Ytti]

Hey Eric,


I'd capture the frames and see what is happening.

42B does not sound like completely arbitrary value. 46B is minimum
payload of L2, and you're adding one label, 4B to it, ending up with
exactly 46B of payload, which is minimum possible payload on ethernet.
Which implies perhaps somewhere MTU is configured below minimum, perhaps 0.

ELAM capture for lookup results at 42B and 43B should be interesting,
perhaps the lookup results are different (they should not be).

You should check what the platform thinks MTU is, by looking at MLS
CEF forwarding, sometimes next-hop MTU is incorrectly programmed, not
same as in CLI. This can usually be fixed by manually changing MTU in
CLI to something arbitrary and then back to what you actually want,
which forces reprogramming of the HW adjacency MTU.

The MTU you see in LDP signalling is completely meaningless, it's just
control-plane signalling, it has nothing to do with forwarding.

You also can't exclude something very odd happening, on ES+ had once
bug on EoMPLS where it would change DADDR in payload by 1 every 5
packets on a /24 border.


On 18 May 2016 at 16:37, Eric Xu <ericxuweili at gmail.com> wrote:
> I have two directly connected Cisco 7609 routers running MPLS with OSPF
> being IGP,  and a EoMPLS VC has been created.
>
> On router 1:
>
> interface Vlan100
> ip address 192.168.168.2 255.255.255.0
> xconnect 10.200.0.158 100 encapsulation mpls
>
> On router 2:
>
> interface Vlan100
> ip address 192.168.168.3 255.255.255.0
> xconnect 10.200.0.157 100 encapsulation mpls
>
> *VC is up on both routers with MTU 1500:*
>
> *router1#sh mpls l2transport vc 100 detail*
> *Local interface: Vl100 up, line protocol up, Eth VLAN 100 up*
> *Interworking type is Ethernet*
> *Destination address: 10.200.0.158, VC ID: 100, VC status: up*
> *Output interface: Te1/0/0.18000064, imposed label stack {16}*
> *Preferred path: not configured*
> *Default path: active*
> *Next hop: 10.200.0.38*
> *Create time: 00:12:14, last status change time: 00:12:14*
> *Last label FSM state change time: 00:12:14*
> *Signaling protocol: LDP, peer 10.200.0.158:0 <http://10.200.0.158:0> up*
> *Targeted Hello: 10.200.0.157(LDP Id) -> 10.200.0.158, LDP is UP*
> *Status TLV support (local/remote) : enabled/supported*
> *LDP route watch : enabled*
> *Label/status state machine : established, LruRru*
> *Last local dataplane status rcvd: No fault*
> *Last BFD dataplane status rcvd: Not sent*
> *Last BFD peer monitor status rcvd: No fault*
> *Last local AC circuit status rcvd: No fault*
> *Last local AC circuit status sent: No fault*
> *Last local PW i/f circ status rcvd: No fault*
> *Last local LDP TLV status sent: No fault*
> *Last remote LDP TLV status rcvd: No fault*
> *Last remote LDP ADJ status rcvd: No fault*
> *MPLS VC labels: local 214, remote 16*
> *Group ID: local 0, remote 0*
> *MTU: local 1500, remote 1500*
> *Remote interface description:*
> *Sequencing: receive disabled, send disabled*
> *Control Word: On (configured: autosense)*
>
> *router2#sh mpls l2transport vc 100 detail*
> *Local interface: Vl100 up, line protocol up, Eth VLAN 100 up*
> *Interworking type is Ethernet*
> *Destination address: 10.200.0.157, VC ID: 100, VC status: up*
> *Output interface: Te2/11.18000064, imposed label stack {214}*
> *Preferred path: not configured*
> *Default path: active*
> *Next hop: 10.200.0.37*
> *Create time: 00:13:06, last status change time: 00:12:53*
> *Last label FSM state change time: 00:12:53*
> *Signaling protocol: LDP, peer 10.200.0.157:0 <http://10.200.0.157:0> up*
> *Targeted Hello: 10.200.0.158(LDP Id) -> 10.200.0.157, LDP is UP*
> *Status TLV support (local/remote) : enabled/supported*
> *LDP route watch : enabled*
> *Label/status state machine : established, LruRru*
> *Last local dataplane status rcvd: No fault*
> *Last BFD dataplane status rcvd: Not sent*
> *Last BFD peer monitor status rcvd: No fault*
> *Last local AC circuit status rcvd: No fault*
> *Last local AC circuit status sent: No fault*
> *Last local PW i/f circ status rcvd: No fault*
> *Last local LDP TLV status sent: No fault*
> *Last remote LDP TLV status rcvd: No fault*
> *Last remote LDP ADJ status rcvd: No fault*
> *MPLS VC labels: local 16, remote 214*
> *Group ID: local 0, remote 0*
> *MTU: local 1500, remote 1500*
> *Remote interface description:*
> *Sequencing: receive disabled, send disabled*
> *Control Word: On (configured: autosense)*
> *SSO Descriptor: 10.200.0.157/100 <http://10.200.0.157/100>, local label:
> 16*
> *Dataplane:*
> *SSM segment/switch IDs: 4144/4142 (used), PWID: 9*
> *VC statistics:*
> *transit packet totals: receive 32, send 15*
> *transit byte totals: receive 2318, send 1020*
> *transit packet drops: receive 0, seq error 0, send 0*
>
> *MTU between the routers are more than 9000:*
>
> *router1#ping 10.200.0.158 size 9000 df-bit source l0*
> *Type escape sequence to abort.*
> *Sending 5, 9000-byte ICMP Echos to 10.200.0.158, timeout is 2 seconds:*
> *Packet sent with a source address of 10.200.0.157*
> *Packet sent with the DF bit set*
> *!!!!!*
> *Success rate is 100 percent (5/5), round-trip min/avg/max = 12/13/16 ms*
>
> *router2#ping 10.200.0.157 size 9000 df-bit source l0*
> *Type escape sequence to abort.*
> *Sending 5, 9000-byte ICMP Echos to 10.200.0.157, timeout is 2 seconds:*
> *Packet sent with a source address of 10.200.0.158*
> *Packet sent with the DF bit set*
> *!!!!!*
> *Success rate is 100 percent (5/5), round-trip min/avg/max = 12/20/44 ms*
>
> *However, MTU on the VC is only 42 bytes, which makes it unusable:*
>
> *router1#ping 192.168.168.3 size 42 df-bit source vlan 100*
> *Type escape sequence to abort.*
> *Sending 5, 42-byte ICMP Echos to 192.168.168.3, timeout is 2 seconds:*
> *Packet sent with a source address of 192.168.168.2*
> *Packet sent with the DF bit set*
> *!!!!!*
> *Success rate is 100 percent (5/5), round-trip min/avg/max = 8/9/12 ms*
>
> *router1#ping 192.168.168.3 size 43 df-bit source vlan 100*
> *Type escape sequence to abort.*
> *Sending 5, 43-byte ICMP Echos to 192.168.168.3, timeout is 2 seconds:*
> *Packet sent with a source address of 192.168.168.2*
> *Packet sent with the DF bit set*
> *....!*
> *Success rate is 20 percent (1/5), round-trip min/avg/max = 8/8/8 ms*
>
> *router1#ping 192.168.168.3 size 44 df-bit source vlan 100*
> *Type escape sequence to abort.*
> *Sending 5, 44-byte ICMP Echos to 192.168.168.3, timeout is 2 seconds:*
> *Packet sent with a source address of 192.168.168.2*
> *Packet sent with the DF bit set*
> *.....*
> *Success rate is 0 percent (0/5)*
>
>
> ESM20G 7600-ES20-10G3C card on router 1 and 7600 ES+ 76-ES+XC-20G3C card on
> router 2.
>
>
> Any ideas why?
>
> Thank you.
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/



-- 
  ++ytti