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.
Single-Topology IPv6 Support
Single-topology IPv6 support on Cisco IOS XR software software allows IS-IS for IPv6 to be configured on
interfaces along with an IPv4 network protocol. All interfaces must be configured with the identical set of
network protocols, and all routers in the IS-IS area (for Level 1 routing) or the domain (for Level 2 routing)
must support the identical set of network layer protocols on all interfaces.
In single-topology mode, IPv6 topologies work with both narrow and wide metric styles in IPv4 unicast
topology. During single-topology operation, one shortest path first (SPF) computation for each level is used
to compute both IPv4 and IPv6 routes. Using a single SPF is possible because both IPv4 IS-IS and IPv6 IS-IS
routing protocols share a common link topology.
Multitopology IPv6 for IS-IS
Multitopology IPv6 for IS-IS assumes that multitopology support is required as soon as it detects interfaces
configured for both IPv6 and IPv4 within the IS-IS stanza.
Because multitopology is the default behavior in the software, you must explicitly configure IPv6 to use the
same topology as IPv4 to enable single-topology IPv6. Configure the
single-topology
command in IPv6 router
address family configuration submode of the IS-IS router stanza.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
205
Implementing IS-IS
Flood Blocking on Specific Interfaces