ISIS Adjacency

Until the lan hellos are received from the neighbor the adjacency remains in the down state. 

When the hellos are received from the neighbor the local router takes a note of the system address of the neighbor

This does not ensure bidirectional communication, then the local router includes the system mac address of the neighbor in the IS neighbor TLV. When the remote router views its own

mac address in the hello message received from the remote end  the adjacency is changed to full. 

Different types of adjacency are given above.

In the case of P2P adjacency the router notes the system id from the source id field from the hello it received from the neighbor and populates this with the neighbor system id field and local circuit id in the extended local circuit id field it changes the neighborship to up.

Neighbor system id and the neighbor extended local circuit id field is important for creating adjacency with the neighbor first the heelo packets are sent

In the above figure router R2 receives the hello but it has not seen its own circuit id in the neighbor ext circuit id field and its own system address in the neighbor system id field since it receives the hello it changes to init state. It notes the neighbor system address from the source id field.

In the above figure the router sees it own system id in the neighbor system id field and its own circuit id in the neighbor system id field and changes the state to up.

CSNP are then sent as seen in the below figure, two CSNP will be sent if the link is L1/L2

ISIS packets-PSNP

Partial Sequence number PDU is similar to CSNP but it contains subset information then the CSNP

This is used to request a specific lsp also in the case of point to point network this is also used for acknowledging a particular LSP as well.

CSNP contains the following:-

Remaining lifetime for the LSP

LSP ID

LSP sequence number and checksum value

The TLV present are authentication TLV and LSP ID TLV.

LSP ID TLV are the most essential part.

ISIS Packet Type:- CSNP

CSNP:-

This is used to maintain consistency in a link state database

These are sent every 10 seconds in a point to point network

If a router receives outdated CSNP it sends the updated LSP

In a broadcast network these are sent by the DIS

CSNP contains the following information:-

Remaining lifetime of the LSP in seconds

LSP ID

LSP checksum

LSP sequence number

If a device receives a CSNP it checks its database if it finds missing information it requests information with the help of PSNP

Packet Length is the size of the CSNP in bytes

Source ID:- This is the system address of the router which originated the packet.

Start LSP ID:- LSP id of the starting LSP

End LSP ID:- LSP ID of the last LSP

Given above is the TLV fields for  CSNP.

Packet capture for  CSNP:-

Isis packet type:- LSP

This is similar to  LSA used in ospf, and is used to exchange routing information.

PDU Length:- This is the length of the lsp in bytes

Remaining Lifetime:- The life time of the LSP in seconds, by default it is 1200 seconds and starts from the highest to the lowest.

LSP ID:- This is the system id+Pseudonode id +number of LSP

Sequence number:- Sequence number of the LSP.

ATT bit:- This is Set when the router is connected to two areas this also known as attachment bit

Checksum:- This sets the checksum.

Partition bit:- This is not used

Overload bit:- If set the receiving router will not use this router for transit this means the originating router has some memory issues

Type of IS:- This indicates the type for the originating router Level 1 or L2.

The Tlv makes isis extendable

Type contains the type present in the value field

Length contains the length of the value field

Value is the data portion of the packet

Area address Tlv

IS neighbor tlv

Interface address tlv :- The ip interfaces of the originating router.

Authentication tlv are the common tlv

IP internal reachability TLV:-  This contains the ip address/masks which are directly connected with this router this also contained the metrics associated with the prefix.

IS reachability:- These contains the neighbors for the originating router and the metrics associated with these neighbors.

A L2 LSP packet also contains the following TLVs-

•  IP External reachability: This TLV lists the IP addresses/ masks external to the routing domain, which can be reached via one of the originating router's interface.

• Inter-domain Routing Protocol Information: This TLV allows L2 LSP to transparently carry information from external routing protocols through IS-IS domain.

QOS: ISIS packet type-- Hello

QOS: ISIS packet type-- Hello: ISIS Packet type hello is used to discover neighbors and after the adjacency is established used as a keep-alive to maintain the adjacencies...

ISIS packet type-- Hello

ISIS Packet type hello is used to discover neighbors and after the adjacency is established used as a keep-alive to maintain the adjacencies.

The first part is the isis hello packet header which is common to all the isis packet.

There are 2 types of hello packets lan hello packets and point to point hello packets.

Lan hello packets are further subdivided to L1 hello packets and l2 hello packets.

Circuit Type:- This indicates the type of router

01 :- L1 router

10:- L2 router

11:- L1/L2 router.

System ID:- This is the source through which the hello packet is received from

Holding Timer:- If the hello packet is not received till the holding timer expires the neighbor is brought down. In ALU the hellos' are sent every 9 seconds and the holding timer is 27 seconds.

PDU length:- This is the length of the hello packets.

Priority:- This is the priority of the Designated IS by default the priority is 64 and the highest priority that can be set is 127.


System ID :- This is the system id plus the pseudonode id

There are also other TLV's present in the hello packet

Area address:- This indicates the area address configured on the router in this case the area address configured is 49.0001 a maximum of 3 area address can be configured, this is usually required in the case of migration purposes.

IS neighbors TLV:- Present only in lan hellos l1 hellos contains the mac address of the l1 neighbors and l2 hellos contains the mac address of l2 neighbors this generally contains the mac address of the neighbors.

Protocols Supported:- This contains the protocols which are supported, for IP it is 81.

IP interface address:- This is the ip address through which the hello PDU is sent.

Padding:- Padding is done to increase the PDU to a minimum PDU.

Three way adjacency are down, init and up

ISIS packet types Hello