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wirelessdog
join:2008-07-15
Queen Anne, MD

wirelessdog

Member

Maxmimum routed hops

This is more of a general question than pertaining to Cisco but here goes.

I have been experiencing some weird issues - CPU spikes to 100% until reboot on devices, loss of Internet connectivity on one segment of the network when I reboot a switch that doesn't touch that segment except by being on the same vlan, etc etc.

I think I need to migrate to routing at each hop of the network whenever possible and eliminate most of the switching that is going on.

That said, how many routed hops can I have internally without creating a problem with the Internet traffic getting pissed because there are too many? I know, that is a horrible non-technical way to ask the question but its been a long night and I'm tired.

TomS_
Git-r-done
MVM
join:2002-07-19
London, UK

2 edits

TomS_

MVM

In theory the maximum number of hops you can have is about 255, because thats the maximum value you can have in the TTL field before it hits 0 and the packet is dropped.

That applies to pure routing.

If you were using something like RIP with (IIRC) default settings, in theory you are limited to 16 hops as RIP wont propagate routes beyond that diameter from the originating router. But I dont know a single ISP that uses RIP as their IGP...

Your topology can be as complicated as you want it to be. Protocols like OSPF will find the best most direct route through it all, using alternate paths as backups if one happens to fail.

Tie in other protocols like RSVP and TE mechanisms and you can use all available paths, with traffic overflowing on to alternate paths as they fill up.

But you sound like you have an issue that needs fixing, not "working around".
HELLFIRE
MVM
join:2009-11-25

HELLFIRE to wirelessdog

MVM

to wirelessdog
Able to diagram out your network wirelessdog See Profile ? Specifically the segment(s) / device(s) that get the
high CPU and the device that reboots? This is just so we can visualize.

We'll also need sanitized configs as well as usual, but you knew that

At a WAG*, you've just rebooted the root switch, and now all your layer 2 devices are recalculating STP,
which MAY affect your layer 3 devices...

My 00000010bits

Regards

*Wild A$$ Guess
wirelessdog
join:2008-07-15
Queen Anne, MD

wirelessdog

Member

Said switch is a Mikrotik which **cough** doesn't support STP. At any rate, it sounds like if I end up with 10 routed hops I won't screw anything up so I'm going to start there. My network is a complex pieced together mess of over 50 hops spread across two states with no diagram. I know, I know.

TomS_
Git-r-done
MVM
join:2002-07-19
London, UK

TomS_

MVM

You'll definitely want to use an IGP like OSPF to handle route propagation. Trying to do a network of that size with static routes will be a death sentence.

battleop
join:2005-09-28
00000

battleop to wirelessdog

Member

to wirelessdog
Sounds like a typical trace route into Charter's network. I've seen offnet customers on Charter with 10 or more hops from the Charter edge to the customer CPE.
HELLFIRE
MVM
join:2009-11-25

HELLFIRE to wirelessdog

MVM

to wirelessdog
said by wirelessdog:

My network is a complex pieced together mess of over 50 hops spread across two states with no diagram. I know, I know.

...to put it bluntly but politely, may be a good idea to fix that first before trying to tackle "what's exactly causing this?"
That situation's akin to looking for a single snowflake in a blizzard.. in the middle of the night.

My 00000010bits

Regards
wirelessdog
join:2008-07-15
Queen Anne, MD

wirelessdog

Member

Which is why I was asking the routed hops question. Now that I have the answer - that even if I have 10+ routed hops from network edge to headend I will not adversely affect the customers I can start tackling that. Once the network is entirely routed it will make life much easier.

Wily_One
Premium Member
join:2002-11-24
San Jose, CA

Wily_One

Premium Member

"Number of hops" is really only a concern if you're using a distance-vector protocol. (e.g. RIP)

Number of routes will affect size of the routing table in link-state protocols (e.g. OSPF), which may impact convergence time and router performance. Here's where you would want to do route aggregation where possible, which may or may not be an option and depends on your address space usage. As HELLFIRE See Profile says, having an accurate diagram will help you figure this out.
wirelessdog
join:2008-07-15
Queen Anne, MD

wirelessdog

Member

This I will work on. For the moment I'm using static routing which as Tom mentioned has all but been a death sentence. That part has been implemented at this point for the most part. I have one other hop I would like to route but I'm looking at a solid 2-3 hours assigning new IP's to make that happen so I think I'll sit tight for a few days and see if the general weirdness goes away.

TomS_
Git-r-done
MVM
join:2002-07-19
London, UK

TomS_

MVM

People have suggested it already, but I would start by drawing out your current network in some form or another to get an idea of how it all fits together in a way that you can see.

From there, and to minimise the number of actual routed hops, you can then start to segment the network in to domains. A domain might be a string of towers that make up an extremity of your network, or maybe a small cluster of towers spoked off one central tower.

Then you could use VLANs between these domains to route between them in a more controlled manner, perhaps forming redundant adjacencies.

Going in to this without visualising it first is a little risky. You could end up doing work that you later want to revert, and that would require extra effort.

Measure twice, cut once, as they say.