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HSRP over an MPLS core and MTU sizes

I have a senario where two router in an HSRP group connected to an MPLS network, where I cannot get 1500 datagrams from Router 3 to Router 1, but can do Router 2.  I think this is causing slow responses when router 1 is active.

All LAN connections are connected at 1Gig.

Is it possible to be able to ping 1500 bytes across the MPLS cloud from routers 2/3 to router 1 using an xconnect?

If so could somebody point in the correct direction? The max size I can get is 1492.

Using the same equipment I can ping 1500 bytes between the two Cisco 3750's togther using L3 and BGP, however I had to change the MTU size on ME ports facing the CE to 1522 for this work.

Setup:-

Router1 2951 (HSRP1) -- Cisco 3750 -- ME3600 -- MPLS Cloud -- ME3800 - Cisco 3750 -- Router2 2951 (HRSP2) --- Router3 (3825).250

CE RTR1                            CE1            PE1            Core          PE2           CE2             C RTR2                               C RTR 3

x.x.x.254                                                                                                                      x.x.x.254

x.x.x.253                                                                                                                      x.x.x.252

The two routers connect and form an HSRP pair and failover works go fine.

The ME3800 and ME3600 are connected using an xconnect command, as below.

ME-3800
interface Vlan100
mtu 1524
no ip address
xconnect x.x.0.1 100 encapsulation mpls

ME-3600
interface Vlan100
mtu 1524
no ip address
xconnect x.x.0.12 100 encapsulation mpls

I have messed around the MTU settings on the xconnect, hence the mtu 1524 above.

The xconnect is UP.

1 ACCEPTED SOLUTION

Accepted Solutions

HSRP over an MPLS core and MTU sizes

Hi Gary

For a L2VPN in VLAN Mode the maximum datagram size which can be sent across an MPLS Backbone configured with default MTU would be calculated after subtracting the MPLS and L2 Headers which would be as below

MPLS 2 Labels = 8 Bytes

L2 Ethernet Frame Header = 14 Bytes

Frame Checksum = 4 Bytes

Vlan Header = 4 Bytes

Hence we would need to subtract 8+14+4+4=30 Bytes

Hence the max datagram size would be 1470 Bytes to be sent across L2VPN

If we need to send 1500 Datagram then we need to set the MTU across the network to be 1530.

Regards

Varma

1 REPLY

HSRP over an MPLS core and MTU sizes

Hi Gary

For a L2VPN in VLAN Mode the maximum datagram size which can be sent across an MPLS Backbone configured with default MTU would be calculated after subtracting the MPLS and L2 Headers which would be as below

MPLS 2 Labels = 8 Bytes

L2 Ethernet Frame Header = 14 Bytes

Frame Checksum = 4 Bytes

Vlan Header = 4 Bytes

Hence we would need to subtract 8+14+4+4=30 Bytes

Hence the max datagram size would be 1470 Bytes to be sent across L2VPN

If we need to send 1500 Datagram then we need to set the MTU across the network to be 1530.

Regards

Varma

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