2. • IS-IS Protocol uses L2 encapsulation Ethernet 802.3/802.2. It do not use
Ethernet II. Ethernet II is used for IP traffic.
• IS-IS Protocol exchanges protocol information by using Link-State PDUs
(Protocol Data Units).
3. • There are four types of PDU packets in IS-IS Protocol.
These IS-IS packets are :
• – IS-IS Hello PDU (IIH)
– Link State PDU (LSP)
– Partial Sequence Number PDU (PSNP)
– Complete Sequence Number PDU (CSNP)
4. • In IS-IS PDUs there is a TLV (Type, Length, Value) field. According to
the need, these fields are added to the PDUs. This TLV field makes IS-
IS extendable and it is the major advantage of IS-IS over OSPF. There are
several TLVs in IS-IS packets. IS-IS Header always same, but according to
the type of the packet, added TLV fields change.
• TLVs are maximum 256 bytes and they contains sub-TLVs.
6. • IS-IS discovers the IS-IS neighbours and IS-IS adjacency via Hello PDUs.
DIS election is also done with this PDUs.
• There are three types Hello PDUs in IS-IS Protocol:
ESH -> ES sent to IS
ISH -> IS sent to ES
IIH -> Between two ISs
7. IS-IS Hellos, ESH,
ISH, IIH
IS-IS Hello interval is 10 seconds and
the dead interval is 30 seconds by
default.
8. • In IS-IS Hello packet, 6 types of data can be included with TLVs. These are:
– Area Addresses TLV
– Padding TLV
– IP Interface Address TLV
– Authentication Information TLV
– Protocols Supported TLV
– Intermediate System Neighbors TLV
9. • IS-IS LSPs are like LSAs in OSPF and contains many information about the
neighbour ISIS Routers and links.
• They are flooded periodically in IS-IS network. L1 and L2 PDUs use
different formats LSPs are stored in IS-IS LSDBs.
• Separate databases are used for L1 and L2 LSPs. Each LSP has sequence
number that shows its version
10. In LSPs, below TLVs are supported
• – Area Addresses TLV
– Originating LSP Buffer Size TLV
– Intermediate System Neighbors TLV
– End System Neighbor TLV
– Authentication Information TLV
– Prefix Neighbor TLV
– Protocols Supported TLV
– IP Interface Address TLV
– IP Internal Reachability Information TLV
– IP External Reachability Information TLV
11. • PDU Type field shows the level of PDU. Level 1 PDU is showed with the
value 18, Level 2 PDU is showed with the value 20.
• ATT bit is the bit that set if the IS is connected to another area.
• OL bit is set to show that the link-state database of IS is overloaded.
• IS Type shows the level of the IS. For level 1, it is 1. For level 2 it is set as 3.
• CSNP (Complete Sequence Number PDU) provides LSP to be sent
reliably. CSNPs are like
12. • Database Description (DD) packets in OSPF and they used to syntonizer the
LSDBs.
• CSNPs are exchange at the beginning, at the router initialization. And every 10
seconds after this, they are sent. In point to point links, both end routers send
CSNPs. But for broadcast networks, only DIS sends CSNPs.
• IS-IS CSNPs carry two types of TLVs . These are :
– Authentication TLVs
– Entries TLVs
13. • PSNPs (Partial Sequence Number PDU) are sent when there is
a missing information. With PSNP, this specific missing information is
requested. And it is also sent for acknowledgement of the receipt. So
PSNPs are like LSAcks and LSRs ( Link State Request) in OSPF.
14. IS-IS CSNPs and PSNPs
• In IS-IS Protocol, PSNPs are
used in point-to-point links.
PSNPs also includes the TLVs
like CLNPs. (The remaining life
of LSP, the ID of LSP, the
Sequence number of LSP,
Checksum Value)
15. Flooding Rules
• IS-IS Protocol compares the new coming LSP and the old one in LSDB
(Link State Database). According to the compartment:
If the version of new coming LSP is newer, then;
• 1. It is installed to the LSDB,
2. PSNP PDU is send as an Ack,
3. This new version flooded to the other neighbors,
4. IS-IS check that if any SPF is needed or not.
19. Same version LSP,
Ack PSNP
• Lastly, if the version of new
coming LSP is same, then;
1. PSNP PDU is send as an Ack
• Sequence Number is used for
checking the version of LSP.
Higher sequence numbers are
newer ones, means new versions.
