RE: BGP TTL check in 12.3(7)T

["Blaine Christian" <blaine.christian () mci com>]

> However, this says a TTL of 254 will be accepted. Now the 
> fact that I  
> can talk to boxes running a slightly older IOS with a TTL of 
> 0 without  
> any problems suggests to me that emitting packets with a TTL 
> of 255 on  
> router A and accepting packets with a TTL of 254 on router B 
> allows for  
> the presence of a router C in the middle. That can't be good.

I suspect they set the limit to 254 because they expected to decrement the
TTL on ingress (or that the box sending the packets would decrement on
send).  You have an interesting point WRT the TTL 0.  Perhaps if you receive
a packet with a TTL of 0 that is destined for yourself you should just
accept it?  It is not clear to me exactly when you "have" to throw away the
packet (on ingress/themiddle/egress).  I believe it is valid to accept a
packet that is destined for yourself with a TTL of zero.

Since I have observed that packets received from some types of routers have
their TTL set to 255 (on upon reaching the receiving device route processor)
I would have to assume that the TTL is not being decremented for route
processor packets.  Of course I was only playing with one router and it may
be vendor dependant (the vendor was not Cisco).

I am not sure that 254 is a good maximum number.  Perhaps someone "in the
know" can enlighten all of us as to why they chose to stop at 254 instead of
255.

Regards,

Blaine