On Thu, 11 Jan 2001, Bulger, Tim wrote:
There is an 'ip mtu' option in the tunnel interface config on Cisco routers that you can set to 1500.. This will cause the tunnel packets themselves to be ip fragmented outbound on the router, hence shifting the fragmentation away from your internal network packets and over to the tunnel packets. I know that this works with GRE and IP/IP on Cisco routers running 12.x code.
(config-if)#tunnel mode gre ip (config-if)#mtu 1500 % Interface Tunnel1 does not support adjustable maximum datagram size I believe this is only true if you have serial links that support MTU > 1500 all the way between the two routers doing the tunneling, right? -- Mikael Abrahamsson email: swmike@swm.pp.se
Thus spake "Mikael Abrahamsson" <swmike@swm.pp.se>
(config-if)#tunnel mode gre ip (config-if)#mtu 1500 % Interface Tunnel1 does not support adjustable maximum datagram size
router(config)#in tun0 router(config-if)#mtu 1500 % Interface Tunnel0 does not support adjustable maximum datagram size router(config-if)#ip mtu 1500 %Warning: MTU set 1500 is greater than default 1476, fragments will happen
I believe this is only true if you have serial links that support MTU
1500 all the way between the two routers doing the tunneling, right?
Nope. With fragmentation of the GRE packet, your tunnel MTU can be greater than your real MTU. S | | Stephen Sprunk, K5SSS, CCIE #3723 :|: :|: Network Design Consultant, GSOLE :|||: :|||: New office: RCDN2 in Richardson, TX .:|||||||:..:|||||||:. Email: ssprunk@cisco.com
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