From: marvin greenlee (marvin@ccbootcamp.com)
Date: Wed Aug 10 2005 - 18:26:10 GMT-3
MTU mismatches are generally not a good thing. The router checks the MTU
when forming the adjacency, and a mismatch can be seen in the output of
debug ip ospf adj
From NLI's Advanced Cisco(r) CCIE(r) R&S Lab Workbook: Volume I, Version
12.2T:
http://www.ccbootcamp.com/advancedccierslabv1.asp
Lab 1 pg 36.
".... When configuring OSPF on the switches, be aware of the possibility of
an MTU mismatch which can cause adjacency formation to fail. The debug
command debug ip ospf adjacency can tell you if there is a mismatch.
Lab1R1#
*May 23 06:52:53.419: OSPF: Rcv DBD from 35.35.35.35 on Ethernet0/0 seq
0x25D5 opt 0x42 flag 0x7 len 32 mtu 1503 state EXSTART
*May 23 06:52:53.419: OSPF: Nbr 35.35.35.35 has larger interface MTU
*May 23 06:52:54.431: OSPF: Send DBD to 35.35.35.35 on Ethernet0/0 seq
0x1352 opt 0x52 flag 0x7 len 32
*May 23 06:52:54.431: OSPF: Retransmitting DBD to 35.35.35.35 on Ethernet0/0
[1]
Possible workarounds include changing the MTU of the interface, or using the
interface command ip ospf mtu-ignore. MTU settings on the switch are saved
separately from the config file, and will survive a reload. They are saved
in the flash file env_vars, which can be viewed with the more command. You
can also use the command show system mtu.
Lab1Cat3550-1#more env_vars
?=
SYSTEM_MTU=1503
Lab1Cat3550-1#
Lab1Cat3550-1#show system mtu
System MTU size is 1503 bytes
Lab1Cat3550-1#
Since the interface that R1 is communicating with is the SVI for VLAN 11, we
will adjust the MTU for the SVI. A global MTU change on the switch would
require a switch reload to take effect.
Lab1Cat3550-1(config)#int vlan 11
Lab1Cat3550-1(config-if)#ip mtu 1500..."
http://www.ccbootcamp.com/advancedccierslabv1.asp
Marvin Greenlee, CCIE#12237, CCSI# 30483
Network Learning Inc
marvin@ccbootcamp.com
www.ccbootcamp.com (Cisco Training)
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Shawn williams
Sent: Wednesday, August 10, 2005 2:03 PM
To: ccielab@groupstudy.com
Subject: OSPF Adjacency issue and MTU mismatch [bcc][faked-from][bayes]
Importance: Low
I'm working with a Cisco 2501 and a Catalyst 3550.
I configured them both for OSPF area 0. I was not able to form an OSPF
adjacency between the two devices until I changed the ethernet MTU size on
the Catalyst 3550 from 1503 to 1500 to match the 2501.
Can someone explain what is happening here? I can pass data between the two
devices without adjusting the MTU size, but can't form an OSPF adjacency. Is
MTU size a parameter that must match like hello timers or does OSPF use the
maximum frame size when forming adjacencies?
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