Solved: OSPF TTL value or multicast TTL Value

lohit prasad · ‎06-17-2014

Hi Team,

As per my knowledge, minimum TTL value should be 2. Then how come OSPF uses TTL =1 to restrict not to forward packet more than one hop ?

Because Lets say R1 >>>> R2 (back 2 back) then when R1 sends TTL =1 with OSPF packet to R2 then R2 will decrea to 1-1=0 and packet will be dropped by R2 right ?

Please help me in understanding.

Nagendra Kumar Nainar · ‎06-17-2014

Hi Lohit,

Because Lets say R1 >>>> R2 (back 2 back) then when R1 sends TTL =1 with OSPF packet to R2 then R2 will decrea to 1-1=0 and packet will be dropped by R2 right ?

<Nagendra>No. When the TTL of received packet reaches 0 after processing, it will still process the packet, if it is destinated to self or any multicast address to which it is listening to.

In this case, all OSPF routers will listen to 224.0.0.5 nd 224.0.0.6. So if it is received with TTL=1, it will be processed further by looking into the payload. But when the TTL reches 0 after processing and if the packet is not self destinated (or to any address to which it is listening), an ICMP error code will be sent to the source.

-Nagendra

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Nagendra Kumar Nainar · ‎06-17-2014

Hi Lohit,

Because Lets say R1 >>>> R2 (back 2 back) then when R1 sends TTL =1 with OSPF packet to R2 then R2 will decrea to 1-1=0 and packet will be dropped by R2 right ?

<Nagendra>No. When the TTL of received packet reaches 0 after processing, it will still process the packet, if it is destinated to self or any multicast address to which it is listening to.

In this case, all OSPF routers will listen to 224.0.0.5 nd 224.0.0.6. So if it is received with TTL=1, it will be processed further by looking into the payload. But when the TTL reches 0 after processing and if the packet is not self destinated (or to any address to which it is listening), an ICMP error code will be sent to the source.

-Nagendra