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Chapter 6 QoS with MPLS in an AVVID-Enabled Network

Considerations for MPLS VPN QoS

After a Link Recovery

After a link recovers:

Routing protocol convergence optimizes the forwarding path after a link recovery.

The label information base might not contain the label from the new next hop by the time the IP
convergence is complete.

End-to-end MPLS connectivity might be intermittently broken after link recovery until IP
reconvergence is complete.

It is possible to use MPLS traffic engineering for make-before-break recovery to reduce or eliminate
broken connectivity after link recovery.

Redundancy

To provide stability for the WAN, you can use redundant links from the MPLS VPN backbone to the
customer site (redundant PE-CE links). To allow for full redundancy, the PE and CE routers can also be
redundant. When this is done, it becomes important to decide how the traffic is routed in steady-state
conditions. The options are:

Using primary-backup links--The links are allocated as primary-backup. Under normal conditions,
no traffic will flow over the backup link. The IGP routes traffic that is outbound from the site to the
MPLS backbone to the primary link using the metrics available. Both CE routers must be receiving
routing updates (either a default route or full routes for the VPN).

When the primary CE router fails, all traffic is routed to the backup CE router and traffic flows
outbound from the backup CE router over the backup link. Return traffic can be routed to the
primary link based on the BGP metrics at the PE routers.

By setting communities and local pref, you can force traffic to the primary PE-CE link in normal
state. Upon failure of the primary CE router or primary CE-PE link, all traffic will flow over the
backup link.

Both links must be configured identically for support of voice and video in the LLQ design (unless
this traffic is forced off-net in the event of a primary route failure). It is good practice to stop all
signalling across the backup as well, to prevent any call setup.

Using load sharing--The links are provided as load sharing. There are several issues with this
option. Admission control and QoS for voice and video become more difficult.

There is no way to effectively ensure that the traffic is fully load-balanced. Therefore, determining
how to create the priority queues on the PE-CE links becomes very difficult. Both CE-PE links must
be configured to support the full load of traffic allowed by the connection admission control
mechanisms of the network.

Both OSPF and EIGRP allow for load sharing by default. However, the MPLS core uses BGP, which
places only a single entry for a route into the routing table, preventing load sharing. To influence
the load-sharing mechanism from the MPLS VPN backbone to the CE devices, the PE routers have
to be configured to prefer some routes over one path and the rest over the other path. While this may
provide a good balance at first, as data flow patterns change over time, this will result in an
unbalanced load between the two links.

Note

For this configuration, there exists a high probability that traffic flows will be asymmetrical.