Network Types

Network Types
One item that makes IS-IS configuration easier than OSPF is that IS-IS supports only two types
of networks instead of four. The supported network types for IS-IS are broadcast multi-access
and point-to-point. Unlike OSPF, the network types for IS-IS are non-configurable. This means
that you’re stuck with whatever network type is assigned to the interface by default.
In order for routers to form adjacencies on broadcast multi-access networks, the router sends
out either a Level 1 LAN Hello PDU or a Level 2 LAN Hello PDU, depending on whether the
router is an L1, L2, or L1/L2 router. On point-to-point networks, the routers send out a pointto-
point Hello PDU.
The network type of broadcast is assigned to all broadcast multi-access interfaces on a
router. For NBMA networks, broadcast is assigned to multipoint subinterfaces and point-topoint
is assigned to all point-to-point subinterfaces. Physical interfaces, which are connected to
NBMA networks, are considered by IS-IS to be multipoint interfaces, so the network type of
broadcast is assigned to them also. It may seem a little confusing, but multipoint WAN connections
are actually treated by IS-IS the same as a broadcast multi-access LAN connection. The
same type of Hello PDUs are used, and a DIS is elected.
There is a serious problem you’ll need to watch for when IS-IS is used over NBMA networks.
That problem is Hello PDU mismatches. This occurs when two devices on the same connection
are sending different types of Hello PDUs. For instance, this problem could arise when you connect
a point-to-point subinterface of one router to a physical interface on another router. If you
remember correctly, the default network type for a physical interface connected to a NBMA network
is broadcast. So what happens in this instance is that you have one router sending Level 1
or 2 LAN Hello PDUs and the other router sending point-to-point Hello PDUs. The two routers
will not form an adjacency.
The way to overcome this issue is to make sure that both sides of the connection are either
point-to-point or multipoint. Here is a list you can memorize so you will not encounter this situation
on NBMA networks:
 Physical interfaces can connect to other physical interfaces or to multipoint subinterfaces.
 Multipoint subinterfaces can connect to other multipoint subinterfaces or to physical interfaces.
 Point-to-point subinterfaces can connect only to other point-to-point subinterfaces.
Configuring IS-IS 219
Now that you have a good understanding of how IS-IS operates, we’ll show you how to configure
IS-IS.