12-23-2003 01:46 PM - edited 03-02-2019 12:31 PM
Hello,
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?
Regards,
Michiel
SWITCH
| (802.1q trunk)
|
| (bridge interface)
ROUTER
: (bridge interface)
:
: (bridge interface)
ROUTER
| (bridge interface)
|
| (802.1q trunk)
SWITCH
---------------------------------------------
version 12.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Router1
!
logging queue-limit 100
!
memory-size iomem 25
ip subnet-zero
!
!
no ip domain lookup
!
ip audit notify log
ip audit po max-events 100
!
interface Loopback0
no ip address
no ip mroute-cache
!
interface Tunnel0
no ip address
tunnel source FastEthernet0
tunnel destination 192.168.1.248
bridge-group 1
!
interface Ethernet0
no ip address
no ip mroute-cache
half-duplex
bridge-group 1
!
interface FastEthernet0
ip address 192.168.0.248 255.255.255.0
no ip mroute-cache
speed auto
!
ip classless
ip route 192.168.1.0 255.255.255.0 192.168.0.253
no ip http server
no ip http secure-server
!
bridge 1 protocol ieee
!
line con 0
line aux 0
line vty 0 4
exec-timeout 15 0
password cisco
login
!
end
version 12.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Router2
!
logging queue-limit 100
enable secret xxxxxxxxx
!
memory-size iomem 25
ip subnet-zero
no ip routing
!
!
no ip domain lookup
!
ip audit notify log
ip audit po max-events 100
!
interface Loopback0
no ip address
no ip route-cache
no ip mroute-cache
!
interface Tunnel0
no ip address
ip access-group 101 out
tunnel source FastEthernet0
tunnel destination 192.168.0.248
bridge-group 1
!
interface Ethernet0
no ip address
no ip route-cache
no ip mroute-cache
half-duplex
bridge-group 1
!
interface FastEthernet0
ip address 192.168.1.248 255.255.255.0
no ip route-cache
no ip mroute-cache
speed auto
!
ip classless
ip route 192.168.0.0 255.255.255.0 192.168.1.253
no ip http server
no ip http secure-server
!
bridge 1 protocol ieee
!
line con 0
line aux 0
line vty 0 4
exec-timeout 15 0
password cisco
login
!
end
12-24-2003 01:43 AM
Hello,
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.
12-24-2003 02:28 PM
I thought that on a 802.1q link the MTU on the bridge interfaces changes of 1500 to 1524, because on a interface the MTU is max 1500 it has to be increased for the trunk.
Is my theorie wrong? Why do you think, changing the MTU isn't necessary on ethernet0, can you please explain that for me
Regards,
Michiel
12-24-2003 05:53 PM
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.
HTH
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