Arista MPLS VPN Step by Step (2c) L2VPN - Type 4 PW, MTU

In the LDP PW configuration, one thing needs to be careful is the MTU. You have to ensure the MTU along the underlying LSP matched. Here is an example:

So we have OSPF and ISIS enabled on VLAN 301/302 on CEs. But only OSPF up, no ISIS neighbor at all

CE1#sh ip os neighbor vrf ce1-pw4-301
Neighbor ID     VRF      Pri State                  Dead Time   Address         Interface
101.255.2.2     ce1-pw4-301 1   FULL/DR                00:00:32    101.101.101.2   Vlan301
CE1#sh isis neighbors vrf ce1-pw4-302
CE1#

Well the ping works as expected, CE1 can ping CE2's vlan 302 address - 102.102.102.2

CE1#ping vrf ce1-pw4-302 102.102.102.2
PING 102.102.102.2 (102.102.102.2) 72(100) bytes of data.
80 bytes from 102.102.102.2: icmp_seq=1 ttl=64 time=0.276 ms
80 bytes from 102.102.102.2: icmp_seq=2 ttl=64 time=0.119 ms
80 bytes from 102.102.102.2: icmp_seq=3 ttl=64 time=0.112 ms
80 bytes from 102.102.102.2: icmp_seq=4 ttl=64 time=0.111 ms
80 bytes from 102.102.102.2: icmp_seq=5 ttl=64 time=0.110 ms

--- 102.102.102.2 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.110/0.145/0.276/0.066 ms, ipg/ewma 0.202/0.208 ms

Both sides configured with MTU 9000

CE1#sh run int vlan 302
interface Vlan302
   mtu 9000
   vrf forwarding ce1-pw4-302
   ip address 102.102.102.1/24
   isis enable isis-ce1-pw4-302

CE2#sh run int vlan 302
interface Vlan302
   mtu 9000
   vrf forwarding ce2-pw4-302
   ip address 102.102.102.2/24
   isis enable isis-ce2-pw4-302

Let's try ping the destination with size of data plus DF-bit

CE1#ping vrf ce1-pw4-302 102.102.102.2 size 9000 df-bit
PING 102.102.102.2 (102.102.102.2) 8972(9000) bytes of data.
--- 102.102.102.2 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 40ms

Ok, it fails but where is the culprit?

PE1 and PE2 are good:

PE1(config-if-Et47/2)#sh ip int brief | grep 88
Ethernet3/1            88.1.11.11/24      up         up              9082
Ethernet45/3           88.3.11.11/24      down       down            9082

PE2(config-if-Et51/1)#sh ip int brief | grep 88
Ethernet33/1           88.4.22.22/24      up         up              9082
Ethernet51/1           88.2.22.22/24      up         up              9082

Let's move on to P routers:

P1#sh ip int brief | grep 88
Ethernet35/1           88.1.4.1/24        up         up              8000
Ethernet51/1           88.1.2.1/24        up         up              8000
Ethernet52/1           88.1.3.1/24        up         up              9082
Ethernet54/1           88.1.11.1/24       up         up              9082

Ok, here is the issue. Let's correct them and ISIS neighbor is up!

CE1#sh isis neighbors vrf ce1-pw4-302

Instance  VRF      System Id        Type Interface          SNPA              State Hold time   Circuit Id
isis-ce1- ce1-pw4- CE2              L2   Vlan302            44:4c:a8:11:6b:e1 UP    8           CE2.02

So, the reason is, the ISIS pads hello packet upto MTU and the oversized packets will be dropped siliently on underlying P routers. Hence ISIS fails to form neighbor. 

The OSPF just negotiates the MTU of connecting interfaces, which are 9000 in both sides, so ospf neighbor is up w/o any issues. But if you have a large LSA database, and ospf utilize the full MTU to transmit it, you will hit this MTU mismatch later, but not in the neigobor discoverage stage.