Traffic Engineering · Topic 3 of 3
Bandwidth Allocation and Bandwidth Limitation
Learn the difference between reserving bandwidth for a TE tunnel and limiting the traffic it carries, and the RouterOS settings that control each of them.
- 4 min
- Allocation reserves bandwidth during RSVP setup, and does not affect packets.
- Limitation restricts real traffic through the tunnel.
- The settings are
bandwidthfor allocation andbandwidth-limitfor limitation.
What it is
Bandwidth allocation reserves capacity on paper, while bandwidth limitation restricts the traffic that really flows.
These two ideas are easy to mix up. The first is a promise made during tunnel setup. The second is an enforcement on the packets that enter the tunnel.
A good design uses both, and the exam expects you to tell them apart.
How it works
Each link announces what it can reserve, each tunnel asks for a share, and a separate limit controls actual traffic.
Bandwidth allocation
Allocation is part of RSVP setup. Each interface in the TE menu states how much bandwidth it makes available for tunnels. A tunnel asks for an amount, and every router on the path subtracts that amount from the interface.
/mpls traffic-eng interface set [find interface=ether1] bandwidth=100M
/interface traffic-eng set te1 bandwidth=40M- The interface
bandwidthis the pool that can be reserved on that link. - The tunnel
bandwidthis the amount the tunnel asks to reserve.
If a router has less free bandwidth than requested, it rejects the request. With a dynamic path, CSPF avoids such links in advance and picks another route.
The remaining free amount is shared with the other routers through the OSPF TE data, so a head end can see how much is left on each link.
The key point about allocation
Allocation is accounting only. It does not slow down any packet. A tunnel that reserved 40M can still send more than 40M, if the link allows.
The reservation only protects the amount in the books, so that the next tunnel cannot reserve what is already promised.
Bandwidth limitation
Limitation acts on real traffic. The tunnel interface has a separate setting that limits what the tunnel carries.
/interface traffic-eng print detailThe setting is bandwidth-limit, found on the tunnel interface next to bandwidth. It limits actual traffic relative to the tunnel bandwidth. Check the console help for its unit and its default before you rely on a figure.
Traffic above the limit is not allowed through the tunnel. This protects the reservations of other tunnels that share the same links.
Side by side
| Point | Allocation | Limitation |
|---|---|---|
| Setting | bandwidth | bandwidth-limit |
| Where it acts | RSVP setup, per link | On the tunnel interface |
| Affects packets | No | Yes |
| Purpose | Prevent over booking of links | Stop a tunnel from using more than its share |
| Seen by other routers | Yes, through RSVP and OSPF | No, it is local to the head end |
Priorities
When capacity is short, a more important tunnel can displace a less important one. Each tunnel has a setup priority and a holding priority. A new tunnel with a better setup priority than the holding priority of an existing tunnel can take its place.
The existing tunnel is then torn down and must find another path. Use this with care, because it causes outages for the displaced traffic.
Checking the result
/interface traffic-eng print
/mpls traffic-eng interface print- The first command shows the tunnels and their state.
- The second shows the TE interfaces and the bandwidth settings.
To see allocation and limitation at once, ask two questions. Can the tunnel be built on this path, and can the tunnel send more than it reserved?
What to remember for the exam
Seven facts cover most questions here.
- Allocation reserves bandwidth during RSVP setup, and does not affect packets.
- Limitation restricts real traffic through the tunnel.
- The settings are
bandwidthfor allocation andbandwidth-limitfor limitation. - The interface
bandwidthin the TE menu is the pool available for reservations. - A router rejects a tunnel request when it lacks free bandwidth.
- CSPF avoids links that cannot meet the requested bandwidth.
- Setup and holding priorities decide which tunnel wins when capacity is short.
A reservation does not stop a tunnel from sending more. Only the limitation setting acts on real traffic.
Common mistakes
These four errors are common.
Thinking a reservation is a rate limit. It is a bookkeeping entry that other tunnels respect.
Reserving more than the interface pool. The router refuses the request, and the tunnel stays down.
Skipping the limit and expecting enforcement. Without it, a tunnel can send more than it reserved.
Using priorities carelessly. A higher priority tunnel can push another one off a link.
Check yourself
Work through these three questions.
What is the difference between bandwidth allocation and bandwidth limitation?
Allocation reserves bandwidth during RSVP setup and does not affect packets. Limitation restricts the real traffic that enters the tunnel.
What happens if a router has less free bandwidth than a tunnel requests?
The router rejects the request, so the tunnel does not come up on that path. With a dynamic path, CSPF avoids such links.
Does a tunnel with
bandwidth=40Msend at most 40M?Not by itself, because the reservation is accounting only. The
bandwidth-limitsetting is what restricts real traffic.
Topic 3 of 3 in Traffic Engineering
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