Cisco NCS 5500 Series

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.
Control LSP Flooding for IS-IS
Flooding of LSPs can limit network scalability. You can control LSP flooding by tuning your LSP database
parameters on the router globally or on the interface. This task is optional.
Many of the commands to control LSP flooding contain an option to specify the level to which they apply.
Without the option, the command applies to both levels. If an option is configured for one level, the other
level continues to use the default value. To configure options for both levels, use the command twice. For
example:
RP/0/RP0/CPU0:router(config-isis)# lsp-refresh-interval 1200 level 2
RP/0/RP0/CPU0:router(config-isis)# lsp-refresh-interval 1100 level 1
SUMMARY STEPS
1. configure
2. router isis instance-id
3. lsp-refresh-interval seconds [ level { 1 | 2 }]
4. lsp-check-interval seconds [ level { 1 | 2 }]
5. lsp-gen-interval { [ initial-wait initial | secondary-wait secondary | maximum-wait maximum
] ... } [ level { 1 | 2 }]
6. lsp-mtu bytes [ level { 1 | 2 }]
7. max-lsp-lifetime seconds [ level { 1 | 2 }]
8. ignore-lsp-errors disable
9. interface type interface-path-id
10. lsp-interval milliseconds [ level { 1 | 2 }]
11. csnp-interval seconds [ level { 1 | 2 }]
12. retransmit-interval seconds [ level { 1 | 2 }]
13. retransmit-throttle-interval milliseconds [ level { 1 | 2 }]
14. mesh-group { number | blocked }
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
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Implementing IS-IS
Limit LSP Flooding