interview question

[sivakondalarao]

Hi,

there are two ospf areas. when trying to ping host in one area from another area, I am getting these errors.

TTL expired in transit.

Request timed out.

If TTL expired in transit. what is the problem?

If Request timed out. what is the problem?

Regards

skrao

[bhedlund]

"TTL expired in transit" happens when there is a routing loop.

"Request timed out" means the echo request did not make it the the destination and/or the the destination was not able to return the echo reply back to the source.

This could happen from a number of things such as links being down, missing routes, access-lists blocking traffic etc.

Please remember to rate all helpful posts.

Regards,

Brad

[sivakondalarao]

Hi,

thanks for ur response. but i don't think TTL expired in transit is caused by routing loop.

why because i have seen that error, on leased lines when there is problem in modem or multiplexer.

regards

skrao

[bhedlund · Accepted solution]

When an IP packet is routed from one interface to another, the IP router will decrement the TTL value found in the IP header. When an IP packet is bouncing between routers in a routing loop, the TTL value will eventually reach 0 and the packet will be dropped. When the router drops a packet due to TTL reaching 0, it will send the ICMP "TTL Expired in transit" message to the source address of the packet. The purpose of the TTL value is to prevent an endless loop that can disable a network.

If a routing protocol is not configured properly, the failure of an interface can cause a routing loop.

Please rate all helpful posts.

Regards,

Brad