RE: Bridging problem with

From: simon hart (simon@harttel.com)
Date: Fri Sep 02 2005 - 17:53:59 GMT-3

Hi Chris,

Yes that cures the problem, but does not help me understand the issue at
play here. What I am trying to understand is why does the Cat appear to
successfully negotiate a trunk on a router port that clearly does not
support trunking?

When the 3550 port is in dynamic desirable, will it immediately convert the
port to an ISL trunk on reciept of BPDU's. This was or has not been my
understanding, however this seems to be the results.

Thanks

Simon

-----Original Message-----
From: Chris Lewis (chrlewis) [mailto:chrlewis@cisco.com]
Sent: 02 September 2005 20:58
To: simon hart; Group Study
Subject: RE: Bridging problem with

Have you looked at the effects of putting

bridge-group 1 spanning-disable

In the router interface configuration?

Chris

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
simon hart
Sent: Friday, September 02, 2005 2:37 PM
To: Group Study
Subject: Bridging problem with

All,

This may be a dumb question but has got me a little confused when
configuring ports on a 3550.

I have the following setup

R3 e0/0-------------------fa0/3 Cat 3550

R3 is connected to the Cat on Fa0/3. Fa0/3 is configured as follows:

interface FastEthernet0/3
 switchport access vlan 363
 switchport mode dynamic desirable

R3 int e0/0 is configure as
interface Ethernet0
 ip address 192.10.1.3 255.255.255.0

If I look at the spanning tree for vlan 363 on the cat, I will see the
following:

Interface Port ID Designated
Port
ID
Name Prio.Nbr Cost Sts Cost Bridge ID
Prio.Nbr
---------------- -------- --------- --- --------- --------------------
----- 

---
Fa0/6            128.6           19 FWD       109 33131 000e.381f.d100
128.6
Fa0/24           128.24         100 FWD       109 33131 000e.381f.d100
128.24
Po1              128.65           9 FWD       100 33131 0009.e8b9.c200
128.65

Now whilst working on a DLSW problem, I put R3's int e0 into
bridge-group 1 (running ieee).  After applying the configuration, I lose
connectivity on the switch.  The spanning tree on the CAT looks like
below:

Interface        Port ID                     Designated
Port
ID
Name             Prio.Nbr      Cost Sts      Cost Bridge ID
Prio.Nbr
---------------- -------- --------- --- --------- --------------------
-----
---
Fa0/3            128.3          100 BLK         0 33131 0009.e8b9.c200
128.3
Fa0/6            128.6           19 FWD         9 33131 000e.381f.d100
128.6
Fa0/24           128.24         100 FWD         9 33131 000e.381f.d100
128.24
Po1              128.65           9 FWD         0 33131 0009.e8b9.c200
128.65

Also the int Fa0/3 is now trunking:

Port      Mode         Encapsulation  Status        Native vlan
Fa0/3     desirable    n-isl          trunking      1

Port      Vlans allowed on trunk
Fa0/3     1-4094

Port      Vlans allowed and active in management domain
Fa0/3     1,4-7,32,77,363,777

I know that if I change the int Fa0/3 to switchport mode access the
trunking will stop, and the span tree will go back to normal (i.e. no
blocking).

My question is how can a trunk form between R3 and the CAT.  R3 is a
2500 using a 10Mbps ethernet interface, and thus no trunking capability,
however the CAT says that it is trunking.  Is this normal behaviour?

Also, when configuring DLSW and putting a bridge statement on an
ethernet interface is it a must to change the Cat port to switchport
mode access?

Any guidance or comments will be greatly appreciated

Thanks

Simon

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