MPLS · Topic 4 of 7
L2MTU, MPLS MTU and Fragmentation
Learn why labels and tunnels need larger frames, how L2MTU and MPLS MTU differ, how to calculate VPLS overhead, and what happens when packets are too big.
- 5 min
- Every label adds 4 bytes, so MPLS and VPLS need larger frames than plain Ethernet.
- L2MTU is the frame size limit of the interface, and the MPLS MTU is the labelled packet limit.
- The L2MTU must be large enough to back the MPLS MTU.
What it is
Every label and tunnel header makes packets bigger, so the links in the core must accept bigger frames.
If a link cannot carry the larger size, packets are fragmented, dropped or the tunnel does not work. This is one of the most common faults in MPLS and VPLS networks.
Three values matter.
- The MTU is the largest layer 3 packet.
- The L2MTU is the largest frame size the interface can handle.
- The MPLS MTU is the largest labelled packet allowed on an MPLS interface.
How it works
Add up the extra headers, compare the total with the MPLS MTU, and make sure the L2MTU is large enough to back it.
The three sizes
| Value | Meaning | Where to see it |
|---|---|---|
| MTU | Largest layer 3 packet on the interface | mtu |
| L2MTU | Largest frame size the interface can handle | l2mtu and max-l2mtu |
| MPLS MTU | Largest labelled packet on an MPLS interface | mpls-mtu |
/interface print detail
/mpls interface print
/mpls interface set [find interface=ether1] mpls-mtu=1522The L2MTU is a property of the interface. Many Ethernet interfaces can be raised up to the value shown as max-l2mtu, and some cannot be raised at all. The L2MTU of the core interface must be large enough to back the MPLS MTU you set.
Calculating the overhead
Take the example of a VPLS tunnel carrying a customer frame with a 1500 byte payload.
| Item | Bytes |
|---|---|
| Customer payload | 1500 |
| Customer Ethernet header | 14 |
| Two MPLS labels, outer and inner | 8 |
| Control word, if used | 4 |
| Total labelled packet | 1522, or 1526 with the control word |
The core link needs an MPLS MTU of at least that total. The L2MTU of the core link must be large enough to back that MPLS MTU. If the customer frames carry a VLAN tag, add 4 bytes more.
A plain MPLS path with a single label adds only 4 bytes to an IP packet. VPLS adds more, because it carries a whole customer frame plus two labels.
What happens when a packet is too big
Behaviour depends on what is being carried.
- An IP packet inside an LSP can be fragmented by a router, if the packet allows fragmentation.
- If the packet is marked do not fragment, the router drops it and sends an ICMP message back.
- A VPLS Ethernet frame cannot be fragmented like an IP packet. If it is too big for the core, it is dropped.
For VPLS the typical symptom is odd. Pings work, small web pages load, but large transfers stall. This happens because small frames fit and large frames do not.
MTU checks between tunnel ends
The two ends of a pseudowire also tell each other the MTU they expect, and a mismatch can keep the tunnel from coming up. For that reason both ends should be set to the same MTU, and the core must support it.
There are two ways to resolve a shortage of size.
- Raise the L2MTU and MPLS MTU on every core link to match the tunnel needs.
- Lower the customer side MTU so the encapsulated frame fits in the existing core limit.
The first is preferred, because the customer then keeps the normal 1500 byte size. Every single link on the path must be raised. One small link anywhere in the path is enough to break large frames.
Test with large pings that have the do not fragment flag set. Increase the size until it fails to find the limit of the path.
What to remember for the exam
Seven facts cover most questions here.
- Every label adds 4 bytes, so MPLS and VPLS need larger frames than plain Ethernet.
- L2MTU is the frame size limit of the interface, and the MPLS MTU is the labelled packet limit.
- The L2MTU must be large enough to back the MPLS MTU.
- VPLS adds two labels, and optionally a 4 byte control word, to a full customer frame.
- A 1500 byte customer payload needs 1522 bytes, or 1526 with the control word, in the core.
- IP packets can be fragmented in an LSP if allowed, but VPLS Ethernet frames cannot.
- Mismatched MTU values at the two ends can keep a pseudowire down.
One core link with a small L2MTU breaks large frames across the whole path, while small pings still pass.
Common mistakes
These four errors are common.
Checking only the MTU. The L2MTU and MPLS MTU decide whether labelled frames fit.
Raising only some links. The smallest link on the path sets the limit.
Forgetting the control word and VLAN tag. Each one adds bytes, so the totals should include them.
Testing only with small pings. They succeed and hide the problem.
Check yourself
Work through these three questions.
How large is a labelled core packet for a VPLS frame with a 1500 byte payload and no control word?
It is 1522 bytes, made up of 1500 payload, 14 customer Ethernet header and two 4 byte labels.
Why is a VPLS frame that is too big a bigger problem than a too big IP packet?
An IP packet in an LSP can be fragmented if the packet allows it. A VPLS Ethernet frame cannot be fragmented that way and is dropped.
What is the typical symptom of too small an MTU in the core?
Small pings and small transfers work, but large transfers stall.
Topic 4 of 7 in MPLS
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