Ip tcp path MTU descovery

[mahesh-gohil]

Dear all,

I have cisco 7206 router with IOS 12.2(25)S5

Now i want to enable one feature Ip tcp path MTU descovery

B'se presently it is MSS value is 536 bytes

Router#sh ip bgp neighbors | inc max data

Datagrams (max data segment is 536 bytes):

Datagrams (max data segment is 536 bytes):

I want to know whether it is configured by default if not then what else i need to configure and after configuring how will it affect

Regards

[mahesh-gohil]

Pl.

Provide some comments

REgards!!

Mahesh

[swaroop.potdar]

The default TCP Maximum Segment Size is 536 as defined in the RFC 879.

Now the formula used to derive this is as below.

MSS = MTU - MinTCLHeadrLen - MinIP HeadrLen = 20 - 20 = MTU - 40.

And its assumed that end Hosts should not send datagrams larger than 576 bytes/Octets.

So 576 - 40 = 536

Now in current scenario the MTU natively is always atleast 1500 bytes.

So the end Hosts have an added option in TCP header to negotiate the

MSS value based on the intermediate min path MTU.

For example:

Router A has default MSS of 536.

Now Router A wants to establish BGP session with 2 routers via different paths.

Path 1 --- RA --1500MTU-- RB --1526-- RC --1500-- RD.

Path 2 --- RA --1526MTU-- RX --1526-- RY --1526-- RZ.

In this case the first session would have an MSS size of Min Path MTU - 40 = 1560

And Path 2 would have Min Path MTU - 40 = 1586.

For this the IP TCP Path MTU discovery needs to be configured on the intermediate

path in a IP network.

In your case I believe this is your native BGP used for MPBGP, and you should not configure anything for this, and the best value possible would be negotiated by the endpoints.

In case if its for a customer then you can configure IP TCP path MTU discovery on your edge termination points.And based on the Edge MTU you need to have a larger MTU facing towards the core

to support the Edge MTU transmission. There are some previous threads on this topic on MTU configuration for End CE.

HTH-Cheers,

Swaroop

[mahesh-gohil]

Dear swaroop,

#sh ip bgp neighbors | inc max data

Datagrams (max data segment is 536 bytes):--to RR

Datagrams (max data segment is 536 bytes):--to RR

Now there is no BGP Customer in ROuter

If i will configure IP tcp path mtu discovery will there two peer will have MSS value increased w.r.t MTU value?

Next do i need to configure at both end or at my end only

Regards!!

[mahesh-gohil]

oNE MORE THING

how can i view details of MSS for customer link b'se customer is configured under ipv4 vrf address family

tell me command for that pl..

regards

mahesh

[swaroop.potdar · Accepted solution]

For End CE's you can check the MSS value on the Edges (CE) itself.

You can configure tcp mtu discovery on the CE also.

And on the PE this command would be configured for BGP scaling and tuning. But the activity of scaling and tuning is not limited to only configuring the MTU discovery. So if you study the hold queue, tcp window size and mtu requirements then you can go ahead and configure the same on the Edge PE for BGP scaling and tuning.

The command to configure the discovery is "ip tcp path-mtu-discovery" in the global.

Which is to be enabled only on the BGP speakers.

HTH-Cheers,

Swaroop

[mahesh-gohil]

Thank you very much for your reply

Regards

Mahesh