Hi,
we are seeing MTU related problems on a link sketched below, I seek comments
from the cisco-nsp-readers if I did something wrong here. See below for a chart.
RtA needs to appear directly connected to the ethernet below (between RtB and RtC)
for various reasons (mainly size of routing table, a full BGP view, RtB is a
small Cisco, RtA is a 3640, RtC is some router that's not administered by me).
So RtA has a BVI and one frame-relay channel in bridge-group 1, RtB has it's ethernet
and the corresponding frame-relay channel in it's bridge-group 1.
-> the BVI should appear as if it was connected to the Ethernet at the bottom,
that one works as expected.
Now I will mention the problem I have: Packets with a IP-packet-size between
1498 and 1500 do not get fragmented but just get lost on their way between RtA
and RtB. (A ping with size =1470 for example), bigger packets get fragmented as
they should (and even icmp fragmentation needed are sent by RtA)
My question to you nsp-readers:
The MTU on RtA's Frame and BVI is 1500, so is RtB's MTU on Ethernet and Frame.
-> Are there two bytes lost when doing the bridge-group's encapsulation over frame?
-> I though about doing the following on the Frame circuits, will this have any
negative side-effects I did not think of? (I cannot imagine any).
interface Serial1/0:0 <- this is the 'main' frame interface
mtu 1600 <- extra padding for bridge-group encaps
interface Serial1/0:0.16 <- this is my ip routing frame dlci
ip mtu 1500 <- to have the 'normal' (=Ethernet) IP mtu
[ Wst ] <-- my Workstation
|
| <-- various other Interfaces
[ ] (BVI1 in BridgeGroup 1 doing IP routing)
[ RtA ]
| <-- E1, FrameRelay DCE
| Ser1/0:0.16 is IP routing
| Ser1/0:0.17 is BridgeGroup 1
...
|
|
(CSU/DSU)
| <-- Serial, FrameRelay DTE
| Serial0:0.16 is IP routing
| Serial0:0.17 is BridgeGroup 1
[ ]
[ RtB ]
| <-- Ethernet in BridgeGroup 1
|
| <-- Ethernet
[ ]
[ RtC ]
Chris
-- Caution. Blade is sharp. Keep out of children.
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