Limit Lsp Flooding; Flood Blocking On Specific Interfaces; Mesh Group Configuration; Maximum Lsp Lifetime And Refresh Interval - Cisco ASR 9000 Series Configuration Manual

Implementing IS-IS

Limit LSP Flooding

Limiting link-state packets (LSP) may be desirable in certain "meshy" network topologies. An example of
such a network might be a highly redundant one such as a fully meshed set of point-to-point links over a
nonbroadcast multiaccess (NBMA) transport. In such networks, full LSP flooding can limit network scalability.
One way to restrict the size of the flooding domain is to introduce hierarchy by using multiple Level 1 areas
and a Level 2 area. However, two other techniques can be used instead of or with hierarchy: Block flooding
on specific interfaces and configure mesh groups.
Both techniques operate by restricting the flooding of LSPs in some fashion. A direct consequence is that
although scalability of the network is improved, the reliability of the network (in the face of failures) is reduced
because a series of failures may prevent LSPs from being flooded throughout the network, even though links
exist that would allow flooding if blocking or mesh groups had not restricted their use. In such a case, the
link-state databases of different routers in the network may no longer be synchronized. Consequences such
as persistent forwarding loops can ensue. For this reason, we recommend that blocking or mesh groups be
used only if specifically required, and then only after careful network design.

Flood Blocking on Specific Interfaces

With this technique, certain interfaces are blocked from being used for flooding LSPs, but the remaining
interfaces operate normally for flooding. This technique is simple to understand and configure, but may be
more difficult to maintain and more error prone than mesh groups in the long run. The flooding topology that
IS-IS uses is fine-tuned rather than restricted. Restricting the topology too much (blocking too many interfaces)
makes the network unreliable in the face of failures. Restricting the topology too little (blocking too few
interfaces) may fail to achieve the desired scalability.
To improve the robustness of the network in the event that all nonblocked interfaces drop, use the csnp-interval
command in interface configuration mode to force periodic complete sequence number PDUs (CSNPs) packets
to be used on blocked point-to-point links. The use of periodic CSNPs enables the network to become
synchronized.

Mesh Group Configuration

Configuring mesh groups (a set of interfaces on a router) can help to limit flooding. All routers reachable over
the interfaces in a particular mesh group are assumed to be densely connected with each router having at least
one link to every other router. Many links can fail without isolating one or more routers from the network.
In normal flooding, a new LSP is received on an interface and is flooded out over all other interfaces on the
router. With mesh groups, when a new LSP is received over an interface that is part of a mesh group, the new
LSP is not flooded over the other interfaces that are part of that mesh group.

Maximum LSP Lifetime and Refresh Interval

By default, the router sends a periodic LSP refresh every 15 minutes. LSPs remain in a database for 20 minutes
by default. If they are not refreshed by that time, they are deleted. You can change the LSP refresh interval
or maximum LSP lifetime. The LSP interval should be less than the LSP lifetime or else LSPs time out before
they are refreshed. In the absence of a configured refresh interval, the software adjusts the LSP refresh interval,
if necessary, to prevent the LSPs from timing out.
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