[nsp] bridge over framerelay MTU problem.

From: Christian Vogel (chris@obelix.hedonism.cx)
Date: Mon Apr 15 2002 - 05:36:50 EDT


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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