I configured two 1710 routers for transparent bridging. The routers are connected with a tunnel, which also part of the bridge. I have tested the configuration and it works oke (see configuration below).
But the routers need to be installed between two switches with trunk interfaces 802.1q (see below). For transparent bridging I thought I increase the MTU on the interfaces. I tried to increase the MTU of the interfaces to 1524 but the router gives the message
% Interface Ethernet0 does not support user settable mtu.
What Im I doing wrong? who can give me some tips?
I dont see any need to change MTU if you introduce 802.1q. What is the motive behind MTU change and where do you want to change that? It should work as usual even if you make routers between switches as you mentioned above.
If it is on fastethernet interface to compensate the GRE overhead (extra 24 bytes). You can give "ip mtu 1524". But I dont think it is necesassary in most of the cases.
I got your point! Please note additional overhead introduced by 802.1q trunking is 4 bytes but not 24 bytes as you said.
In cisco terminology MTU size does not include link layer overhead. on ethernet interface if MTU is 1500 bytses, that means maximum frame size supported is 1518 bytes (Etehret header+FCS). When you enable 802.1q, this makes frame size (Maximum) 1522.
Usually on switches (almost all) can accommodate this additional 4 bytes when you enable trunking on interface. same way when you enable inter VLAN routing with external router (router on stick), support for baby giants (Frame size of up to 1522) is assumed.
If you want to try this conenct 2 PCs in 2 different VLANs and ping with data payload of 1472 (Enable Inter VLAN routing for test sake)
ping -t -f -l 1472 "ip address"
Above makes 1500 Bytes in total (1472+8 Byte ICMP header+20 byte IP header)...If it is not through, may be problem with the platform. Please note, it is quite logical to think baby giant frames are also supported when 802.1q is supported.
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