What is the default MTU values for Cisco 7507 and 7505 routers?

Unanswered Question
mohammedmahmoud Tue, 05/22/2007 - 03:10

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.

David Salazar Mon, 05/25/2009 - 06:51

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.

Giuseppe Larosa Mon, 05/25/2009 - 08:01

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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