05-22-2007 01:59 AM - edited 03-03-2019 05:05 PM
Hi,
I am using Cisco 7507 and 7505 access routers for my internet. What is the default MTU values for ATM links if i do not configure any MTU settings in my interface.
Thanks in advance.
Tony
05-22-2007 03:10 AM
Hi,
Cisco ATM router interfaces support an MTU between 64 and 17966 bytes. Most Cisco ATM router interfaces use a default MTU size of 4470 bytes. This number was chosen to match exactly Fiber Distributed Data Interface (FDDI) and High-Speed Serial Interface (HSSI) interfaces for autonomous switching.
NOTE: The MTU is a function of the interface not the router, for example, the maximum value is 9288 bytes on both the ATM interface processor (AIP) and network processor module (NP), and 4470 bytes on the PA-A3 and PA-A2 port adapters (which are most probably the interfaces you are using on your 7500).
HTH, please do rate all helpful replies,
Mohammed Mahmoud.
05-22-2007 05:11 PM
Oh great!! Is there any cisco reference or documentation to support on your advise?
Thanks!!
05-22-2007 10:45 PM
Hi,
Sure, check this out "Understanding Maximum Transmission Unit (MTU) on ATM Interfaces":
http://www.cisco.com/en/US/tech/tk39/tk371/technologies_tech_note09186a00800c8279.shtml
HTH, please so rate all helpful replies,
Mohammed Mahmoud.
05-25-2009 06:51 AM
I had a Router 7513 with ATM`s PortAdapter on VIP 6-80
Router 7513
NAME: "Line Card 12", DESCR: "Versatile Interface Processor (VIP6-80)"
PID: VIP6-80 , VID: Hardware Version : 2.03, Board Revision : B0, SN: 33021650
NAME: "Card Slot 12, Bay 0", DESCR: "ATM WAN OC3 MM Port Adaptor"
PID: PA-A3-OC3-MM , VID: Hardware Version : 2.0, Board Revision : A0, SN: 24489952
NAME: "Card Slot 12, Bay 1", DESCR: "ATM WAN OC3 MM Port Adaptor"
PID: PA-A3-OC3-MM , VID: Hardware Version : 2.0, Board Revision : A0, SN: JAE08512HQ0
With this configuration:
interface ATM12/0/0
description Red Wan ATM PDVSA
mtu 9188 ---->>>>>>> (((MTU 9188)))
bandwidth 155000
no ip address
ip access-group SNMP in
logging event atm pvc state
logging event subif-link-status
load-interval 30
no atm ilmi-keepalive
cdp enable
!
interface ATM12/0/0.38 point-to-point
description Enlace PLC-Intevep
bandwidth 10642
ip address 200.1.147.14 255.255.255.252
ip bandwidth-percent eigrp 4944 5
pvc 0/38
vbr-nrt 10642 10642
oam-pvc manage
encapsulation aal5snap
transmit-priority 3
max-reserved-bandwidth 95
!
This router recently has a problem with other VIP on Slot0 and the enterprises Management's people request for us to change the links over ATM interfaces on Slot12 to a Router 7613 with
Router 7613
NAME: "3", DESCR: "WS-X6582-2PA 2 port adapter Enhanced FlexWAN Rev. 2.3"
PID: WS-X6582-2PA , VID: V07, SN: JAE102761NB
NAME: "module in bay 0, slot 3", DESCR: "ATM WAN OC3 MM Port Adaptor"
PID: PA-A6-OC3-MM , VID: , SN: JAE10296WW9
NAME: "module in bay 1, slot 3", DESCR: "ATM WAN OC3 MM Port Adaptor"
PID: PA-A6-OC3-MM , VID: , SN: JAE103185C6
We try to stay the same configuration but EIGRP adjacency was flapping; the messages "retry limit exceeded" of EIGRP Neigbors Logs
interface ATM3/0/0
description Red Wan ATM PDVSA
mtu 9188 ---->>>>>>> (((MTU 9188)))
bandwidth 155000
no ip address
ip access-group SNMP in
logging event atm pvc state
logging event subif-link-status
load-interval 30
no atm ilmi-keepalive
cdp enable
!
interface ATM3/0/0.38 point-to-point
description Enlace PLC-Intevep
bandwidth 10642
ip address 200.1.147.14 255.255.255.252
ip bandwidth-percent eigrp 4944 5
pvc 0/38
vbr-nrt 10642 10642
oam-pvc manage
encapsulation aal5snap
transmit-priority 3
max-reserved-bandwidth 95
!
When I delete all configuration for ATM interfaces (default int ATM 3/0/0; no int ATM3/0/0.38) and configure without MTU 9188 command everything work well.
Why is the reason? The other side don`t need to change anything...! I change the FIBER patch coord and the link work on 7513 with ATM MTU Interface of 9188.
05-25-2009 08:01 AM
Hello David,
I strongly suggest you to use a reduced MTU like 1500 bytes.
We had big issues on C7507 caused by the hardware implementation:
all packets have to transit through MEMD specialized buffer that is on the RSP processor even if VIP linecards are involved.
About the reasons why the behaviour is different on C7613 with flexwan linecard it is related to the fact that flexwan allows to use WAN PAs on a C7600 that has a totally different architecture 9188 bytes can be too much to interface to system, so possible problems of efficiency inside the switch could originate from this.
Hope to help
Giuseppe
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