I got this question from a co-worker and I don't have a good explaination for it.
When looking at "Show itnerface tunnel #" what/where does the first MTU refer to (in red) and where does it pull that default from? On physical interfaces it is the physical MTU, on tunnels generally I pay attention to the tunnel MTU (in blue), but we noticed the first MTU in red ... nothing is manually set on our side, brand new devices out of box connecting to a telco managed router, and that is a rather large number. Any ideas?
Tunnel3 is up, line protocol is up
Hardware is Tunnel
Description:
Internet address is 10.200.70.1/24
MTU 17900 bytes, BW 100 Kbit/sec, DLY 50000 usec,
reliability 255/255, txload 255/255, rxload 255/255
Encapsulation TUNNEL, loopback not set
Keepalive set (3 sec), retries 3
Tunnel source 192.168.255.249 (Vlan201), destination 192.168.254.2
Tunnel Subblocks:
src-track:
Tunnel3 source tracking subblock associated with Vlan201
Set of tunnels with source Vlan201, 3 members (includes iterators), on interface <OK>
Tunnel protocol/transport GRE/IP
Key disabled, sequencing disabled
Checksumming of packets disabled
Tunnel TTL 255, Fast tunneling enabled
Tunnel transport MTU 1476 bytes
Tunnel3 is up, line protocol is up
Hardware is Tunnel
Description: MPLS connection to Carolina
Internet address is 10.200.70.1/24
MTU 17900 bytes, BW 100 Kbit/sec, DLY 50000 usec,
reliability 255/255, txload 255/255, rxload 255/255
Encapsulation TUNNEL, loopback not set
Keepalive set (3 sec), retries 3
Tunnel source 192.168.255.249 (Vlan201), destination 192.168.254.2
Tunnel Subblocks:
src-track:
Tunnel3 source tracking subblock associated with Vlan201
Set of tunnels with source Vlan201, 3 members (includes iterators), on interface <OK>
Tunnel protocol/transport GRE/IP
Key disabled, sequencing disabled
Checksumming of packets disabled
Tunnel TTL 255, Fast tunneling enabled
Tunnel transport MTU 1476 bytes
