Chapter 16: Dynamic Routing Protocols
STANDARD Revision 1.0
C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved.
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Point-to-Point Advantages
Compared with broadcast circuits, point-to-point circuits afford more straightforward IGP operation. Specifically, there is
no designated router involved, and there is no representation of the pseudonode or network Link State Advertisement
(LSA) in the link state database.
For IS-IS, there also is no periodic database synchronization which results in improved network convergence performance.
Conversely, if there are more than two routers on the LAN media, the traditional view of the broadcast circuit will reduce
the routing information in the network. When there are only two routers on the LAN, it makes more sense to treat the
connection between the two routers as a point-to-point circuit.
Maintaining IS-IS Point-to-Point Adjacency
The C4/c CMTS maintains IS-IS point-to-point adjacency by supporting both:
IS-IS Point-to-Point Operation over LAN in Link State Routing Protocols (RFC 5309).
Three-Way Handshake for IS-IS Point-to-Point Adjacencies (RFC 5303).
Point-to-Point Operation over LAN — IS-IS Point-to-Point operation over LAN circuit extension is mainly concerned with
pure IP routing and forwarding. Because the circuit physically is broadcast, the IS-IS protocol packets need to have MAC
addresses. From a link-layer point of view, those packets are IS-IS LAN packets.
IS-IS uses Level 1 Hello packet (PDU type 15) and Level 2 Hello packet (PDU type 16) when it is configured for a LAN
environment. However, the protocol uses only Point-to-Point Hello packet (PDU type 17) for both Level 1 and Level 2
adjacencies.
With the Point-to-Point over-LAN extension, the difference between a LAN and a point-to-point circuit can be made purely
by configuration. The C4/c CMTS implements the mechanisms for early detection of misconfiguration. Specifically:
If the circuit is configured as the point-to-point type and receives LAN Hello packets, the router must discard the
incoming packets.
If the circuit is a LAN type and receives point-to-point hello packets, it must discard the incoming packets.
If the system ID or the router ID of an incoming hello packet does not match the system ID or the router ID for an
established adjacency over a Point-to-Point over-LAN circuit, the packet must be discarded.
Both routers on a LAN must support the Point-to-Point over-LAN extension and both must have the LAN segment
configured as a Point-to-Point over-LAN circuit for successful operation. The C4/c CMTS must form adjacency and exchange
routes when both the C4/c CMTS and remote router are configured for Point-to-Point.
