Headers

Jan 15 2009

Frame-Relay LMI

What is LMI?

LMI stands for Local Management Interface.

LMI messages are exchanged between routers(DTE) and frame-relay(DCE) switches.  At its very basic we can say the LMI messages act as keepalives between the router and frame-relay switch.  LMI messages also contain information which allows routers to detect new PVCs dynamically.  Cisco IOS currently supports 3 types of LMI Cisco, ANSI Annex D and Q.933-A.  In order for LMI to work you must use the same LMI type on both the router and frame-relay switch.

All three LMI types listen for LMI messages on reserverd DLCIs.

ANSI and q.933 listen for messages on DLCI 0 and Cisco LMI listens for messages on DLCI 1023.

Consider the topology below.

lmi

As you can see the LMI needs to match between the local DTE and the DCE ie they need to be speaking the same language.

Cisco IOS actually supports an LMI autosense feature which allows a router(DTE) to dynamically detect the LMI type configured on the connected frame-relay switch.

Also worth noting on the diagram.  I preconfigured a PVC on the frame-relay switch with a DLCI if 102 facing R1.  From the debug information you can see that this DLCI is passed to R1.

By Zarar • CCNP, FTP, Headers, Uncategorized • 1

Oct 25 2008

ISIS header

Below is a diagram of the ISIS header.  The different ISIS packets ie hellos and SNPs have slightly different headers, however the first 8 bytes are identical.

Click here to see a power point show on the isis header.

By zismail • CCIE SP, Headers, MPLS • 0 • Tags: , ,

Oct 25 2008

MPLS Header

This is what an MPLS label looks like.  The MPLS label contains a 20-bit label, dont get confused between the different uses of the word label.

The 3 bit EXP bits are used for QoS purposes.  You can use them to define different tunnel modes, which I’ll talk about on another day.

The S bit identifies the bottom label of the stack.

The TTL field is similar to the IP TTL, ie if the MPLs TTL reaches zero the MPLS frame is discarded.

The MPLS frame is inserted between the layer 2 frame header and the layer 3 header.

Click here if you want to see a powerpoint version of the MPLS frame header

By zismail • CCIE SP, Headers, MPLS • 0

Oct 25 2008

IP Packet Header

Below is a diagram of the IPv4 header.  The header is 20bytes long in most cases, however this can be extendend using the options field.

Click here to see a power point version of the header.

By zismail • CCIE, Headers • 0