|
The following information is included in
IIH PDUs:

- Type of PDU – whether the PDU
is a point-to-point WAN PDU or a LAN PDU.
- Source ID – System ID of the
sending router
- Holding time – Time period to wait to
hear a Hello before declaring the neighbor dead. Similar to the OSPF
dead interval, the default value is three times the Hello interval but
can be changed with the isis Hello-multiplier command.
- Circuit type – Circuit type
indicating whether the interface on which the PDU was sent is Level 1,
Level 2, or Level 1 and Level 2.
- PDU length – Length of PDU
packets
- Circuit ID – Local circuit ID
on the sending interface, in point-to-point Hello PDUs
- LAN ID – System ID of the
Designated Intermediate System
(DIS) plus the pseudonode ID, one-byte
circuit ID, to differentiate LAN IDs on the same DIS. On broadcast multiaccess media, LANs, a DIS is elected and will conduct the
flooding over the media. The DIS is analogous to the Designated Router
in OSPF, even though the election process and the definition of
adjacencies on multiaccess media differ significantly. The DIS is
elected by priority, the highest priority becomes the DIS. Priority
is configurable on an interface basis. In the case of a tie, the
router with the highest SNPA address will become the DIS. Unlike OSPF,
there is no backup DIS. The DIS and pseudonode are discussed in
Section 7.3.7.
- Priority – Higher priority
takes precedence. Used in DIS election in LAN Hello PDUs. There is no
DIS election on a point-to-point link.
IS-IS LAN Hello fields are described as follows:
- Intradomain Routing Protocol
discriminator – The network layer identifier assigned to IS-IS in ISO
9577. The binary value is 10000011 (0x83).
- Length indicator – This is the length
of the fixed header in octets.
- Protocol ID Ext – It currently
has value of one (1).
- ID length – Length of the system ID
field. This must be the same for all nodes in the domain. If this is
set to zero, it implies six octets.
- PDU Types – Values are 15 and 16 for
Level 1 and Level 2 LSPs, respectively.
- Version – Value is one (1).
- Maximum area addresses – Number of area
addresses permitted for this IS area. Values are between 1 and 254 for
actual number. Zero implies maximum of three.
- Reserved/circuit type – Top six bits
reserved. Bottom two bits with a value = 0 indicates reserved, with a value = 1
indicates Level 1, with a value = 2 indicates Level 2, and with a value = 3 indicates
Level 1 and 2.
- Source ID – System ID of transmitting
router.
- Holding time – Holding time as
configured on this router.
- PDU length – Length of the entire PDU,
fixed header, and TLVs.
- Reserved/priority – Bit eight (8)
reserved. Bit one (1) is used for priority for being the Level 1 or
Level 2 DIS. Value is copied from the IIH of the DIS.
- LAN ID – A field composed of the system
ID of the DIS, one to eight bytes, plus a low order octet assigned by the
LAN Level 1 DIS.
Notice the variable type length fields at the bottom of the packet.
This is where the TLV information is stored. Different types of PDUs
have a set of currently defined codes. Any codes that are not
recognized are supposed to be ignored and passed through unchanged.
By default, IS-IS Hellos are padded to the full maximum transmission
unit (MTU) size. The benefit of padding IIHs to the full MTU is the
early detection of errors caused by transmission problems with large
frames or MTU mismatches on adjacent interfaces. The drawbacks of IIH
padding are, on high-speed interfaces it could be a strain on huge
buffers and on low-speed interfaces large Hello PDUs waste bandwidth. This could
affect time-sensitive applications such as voice over IP (VoIP). The
padding of IS-IS Hellos can be turned off for all interfaces on a
router, beginning with IOS Release 12.0(5)T, with the
no Hello padding
command in IS-IS router configuration mode. The padding of IS-IS Hellos
can be turned off selectively for point-to-point or multipoint
interfaces with the no Hello
padding multipoint or the
no Hello padding point-to-point
command, respectively, in IS-IS
router configuration mode. Hello padding can also be turned off on an
individual interface basis using the
no isis Hello padding
interface configuration command.
|
Interactive Media Activity
Drag and Drop: IS-IS hello messages
Upon completion of this activity, the student will be able to
identify the fields in an IS-IS LAN Hello PDU.

|
|
|
|