Configuring Frame Mode MPLS
Three steps are required to configure a router to run MPLS:
Step 1. Enable CEF.
Step 2. Enable MPLS on the desired interfaces.
Step 3. Optionally adjust the interface maximum transmission units
(MTU).
This following subsections examine each of these steps more closely.
Enabling CEF
CEF can be enabled either for all interfaces or on individual interfaces only.
If it is not already enabled by default, use the global command ip cef
[distributed] to enable it on all interface. The distributed keyword is used
to enable distributed CEF on line cards or processors capable of running it.
To enable CEF on an individual interface, use the ip route-cache cef interface
command. Note that CEF is not supported on logical interfaces, such as
loopbacks.
Verify CEF operation with any of several variables of the show ip cef
command. The show ip cef detail option is good to start with because it
shows a summary of CEF routes and a list of all adjacencies.
Enabling MPLS
MPLS is globally enabled by default on Cisco routers. If it has been
disabled, enable it with the global command mpls ip. It must then be
enabled on each interface that will participate in label switching; enable it
with the interface command mpls ip.
LDP is the default label distribution protocol in Cisco IOS Release 12.4(3)
and later. There is an older Cisco prestandard version called Tag Distribution
Protocol (TDP.) You might need to enable it if you are connecting to a Cisco
router that does not support LDP; if you need to specify the protocol, use the
interface command mpls label protocol [tdp | ldp | both]. Both protocols
can run on the same router, and even on the same interface. In some Cisco IOS
versions, MPLS commands show in the running configuration as “tag-switching”
commands.
Increasing the MTU Size
The MPLS Frame mode tag adds 4 bytes to every frame. You might need to
increase the interface MTU size to accommodate this, to prevent packets from
being fragmented. The MTU is automatically increased on WAN interfaces,
but must be manually configured on LAN interfaces.
Ethernet uses a default MTU of 1500 bytes. If you are using an MPLS
implementation that uses just one label, increase it to 1504 bytes. MPLS
VPNs and MPLS traffic engineering (TE) use two labels, so you must
increase the MTU size to 1508 bytes if you are using either of these.
Increase the MTU to 1512 bytes when using MPLS VPNs with TE, to
accommodate three headers.
To manually set MTU size, use the mpls mtu bytes command in interface
configuration mode. You may also need to enable jumbo frame support on
the connecting switch.