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| 7.3 |  | IS-IS Operation |
| 7.3.3 |  | IS-IS Hello messages |

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.