In answer to the question, yes the command is available under the eigrp
process. Or at least it was when we were discussing this issue somewhere
around a year ago. ;->
Hey, PO, the only way that the hop count makes any sense is in terms of
relating to the TTL You probably don't recall the thread, but I had found a
document on EIGRP on CCO that made some bizarre claim about the EIGRP max
diameter being 225 because of TCP limitations.
In any case, rightly or wrongly I have linked the max (E)IGRP diameter as
255 because of the TTL. This may or may be a good reason for linking them.
On the other hand, according to RFC 1812 ( unless I am mis-remembering my
reading ) is that required router behaviour is to take any incoming packet,
decrement the TTL by 1, and if the TTL value is still greater than zero,
forward that packet. Otherwise, drop it. In the end, routing protocol
packets are still IP packets, aren't they? Damn, now I have to break out my
RFC's and go through this again!
There is nothing in the (E)IGRP algorithms that calculate hop count, is
there? Bandwidth, delay, reliability, load, MTU. I suppose that somewhere in
the process the number of links crossed as well as the bandwidth of each of
those links figures into the calculations.
Let me ask the question with regards to another routing protocol - does OSPF
have a hop count limit? If so, why? If not, why not?
Come to think of it, most of the routing protocols have a TTL of 1 in the
routing packet. With BGP one can manipulate this using the ebgp-multihop
command. I can think of only one place to do TTL, and that's in the ip
header. Debug ip packet seems to yield information about routing events as
well as data events, does it not?
Where's that bad boy Bob Vance when we need him? This discussion is right up
his alley! ;->
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Priscilla Oppenheimer
Sent: Wednesday, April 25, 2001 2:25 PM
To: [EMAIL PROTECTED]
Subject: RE: TTL [7:1700]
At 08:22 PM 4/24/01, Chuck Larrieu wrote:
>I'm not sure I understand the question, or the need.
>
>The TTL field is 8 bits, which gives a max value of 255.
>
>At this point, I believe that one's only options for manipulating TTL would
>be in terms of changing default hop count maximum for IGRP and EIGRP
routing
>protocols. I.e. the default distance for an (E)IGRP network is 100 ( 100
>hops ) and this can be changed to a max of 255
I would add that changing the hop count wouldn't actually manipulate the
TTL at all. The hop count value affects the routing process and helps a
router decide whether it should advertise a route. The TTL affects the
forwarding process and helps a router decide whether it should forward a
packet.
With IGRP, the Cisco IOS software enforces a maximum diameter to the IGRP
network. Routes whose hop counts exceed this diameter are not advertised.
The default maximum diameter is 100 hops. The maximum diameter is 255 hops.
To configure the maximum diameter, use the following command in router
configuration mode:
Router(config-router)#metric maximum-hops hops
Interestingly enough, this command is not mentioned in the EIGRP
documentation. It may not be supported or necessary in EIGRP. Anyone know?
Priscilla
>RFC 1812 specifies that all packets be dropped after the TTL is expired.
>This is to keep packets from floating around forever. The process is
>specifically described in terms of router behaviour.
>
>I suppose that various applications may manipulate the TTL value in various
>ways, but the fact is that the max value is still 255, and router stacks
are
>generally not configurable in the way you suggest.
>
>Am I understanding your question correctly?
>
>Chuck
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
SH
>Wesson
>Sent: Tuesday, April 24, 2001 6:29 AM
>To: [EMAIL PROTECTED]
>Subject: TTL [7:1700]
>
>If I wanted to increase the TTL on a Cisco router, how can I do that?
>
>Thanks.
>_________________________________________________________________
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________________________
Priscilla Oppenheimer
http://www.priscilla.com
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