Control Plane MTU

By default the control-plane MTU is 1300 bytes inside a VRF (it is 1500 bytes by default in the default routing table):

r4#ping vrf ABC 10.0.0.1 size 1500 df-bit 
PING 10.0.0.1 (10.0.0.1) 1472(1500) bytes of data.
ping: local error: message too long, mtu=1300
ping: local error: message too long, mtu=1300
ping: local error: message too long, mtu=1300
ping: local error: message too long, mtu=1300
ping: local error: message too long, mtu=1300
^C
--- 10.0.0.1 ping statistics ---
5 packets transmitted, 0 received, +5 errors, 100% packet loss, time 40ms

r4#bash

[user@r4 ~]$ sudo ip netns exec ns-ABC ip link show dev fwd0
81: fwd0@if10: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1300 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether c4:ca:2b:aa:bb:cc brd ff:ff:ff:ff:ff:ff link-netns default

[user@r4 ~]$ sudo ip netns exec ns-ABC ip link set mtu 1500 fwd0

[user@r4 ~]$ sudo ip netns exec ns-ABC ip link show dev fwd0
81: fwd0@if10: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether c4:ca:2b:aa:bb:cc brd ff:ff:ff:ff:ff:ff link-netns default

[user@r4 ~]$ exit
logout

r4#ping vrf ABC ip 10.0.0.1 size 1500 df-bit source lo1
PING 10.0.0.1 (10.0.0.1) from 10.0.0.0 : 1472(1500) bytes of data.
1480 bytes from 10.0.0.1: icmp_seq=1 ttl=61 time=4.79 ms
1480 bytes from 10.0.0.1: icmp_seq=2 ttl=61 time=4.77 ms
1480 bytes from 10.0.0.1: icmp_seq=3 ttl=61 time=4.76 ms
1480 bytes from 10.0.0.1: icmp_seq=4 ttl=61 time=4.76 ms
1480 bytes from 10.0.0.1: icmp_seq=5 ttl=61 time=4.77 ms

--- 10.0.0.1 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 20ms
rtt min/avg/max/mdev = 4.761/4.769/4.790/0.010 ms, ipg/ewma 5.001/4.779 ms

This does not persist across reboots.

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