Re: looping traceroutes
At 09:31 PM 9/18/2001 -0700, Ratul Mahajan wrote: [SNIP]
28 193.251.133.117 (193.251.133.117) 586.776 ms 584.171 ms 586.944 ms 29 193.251.133.117 (193.251.133.117) 608.677 ms 610.789 ms 618.232 ms 30 193.251.133.117 (193.251.133.117) 641.141 ms 643.149 ms 639.050 ms
This sometimes happens when there is a filter on that router. All packets going through get an ICMP administratively denied sent back, and the traceroute host interprets that as an ICMP TTL expired or perhaps does not even notice what type of ICMP it is, just looks at the source IP address.
27 208.63.128.3 (208.63.128.3) 111.475 ms 69.670 ms 69.267 ms 28 208.63.128.1 (208.63.128.1) 68.883 ms 67.147 ms 72.106 ms 29 208.63.128.3 (208.63.128.3) 69.842 ms 67.889 ms 66.944 ms 30 208.63.128.1 (208.63.128.1) 70.986 ms 73.124 ms 68.452 ms
This looks exactly like a routing loop to me. Why do you think it is not a routing loop? -- TTFN, patrick
27 208.63.128.3 (208.63.128.3) 111.475 ms 69.670 ms 69.267 ms 28 208.63.128.1 (208.63.128.1) 68.883 ms 67.147 ms 72.106 ms 29 208.63.128.3 (208.63.128.3) 69.842 ms 67.889 ms 66.944 ms 30 208.63.128.1 (208.63.128.1) 70.986 ms 73.124 ms 68.452 ms
This looks exactly like a routing loop to me. Why do you think it is not a routing loop?
I was assuming that any routing loop in the system would be transient, but this one is not (the traceroute is still showing the same behavior). The extent to which I see this also makes me think that there is something else going on. Do you think that it can be a real persistent routing loop (data packets would actually shuttle between the two interfaces) as against some wierdness because of traceroute? thanks, -- ratul
[...] I was assuming that any routing loop in the system would be transient, but this one is not (the traceroute is still showing the same behavior).
That is not a good assumption to make.
The extent to which I see this also makes me think that there is something else going on.
It is, indeed, something else.
Do you think that it can be a real persistent routing loop (data packets would actually shuttle between the two interfaces) as against some wierdness because of traceroute?
Yes. The case of an upstream following a route to a prefix that the downstream entity does not cover is, possibly, the most common instance of this. For example, the upstream router points a static for 192.168.1.0/24 at the downstream router. The downstream router subnets that /24 into two prefixes: 192.168.1.0/25, and 192.168.1.128/26; the remaining /26 is "reserved for future growth," but the operator does not install a covering route for the whole /24. The downstream router points a static default at the upstream router. Now, if a packet is addressed to a destination in the "missing" /26, it will follow the /24 route to the downstream router, find no route there more specific than default and go back to the upstream router, follow the /24 route to the downstream router, find no route there more specific than default and go back to the upstream router, etc., until TTL expires. Traceroute will display the loop for you, as you have seen. Since the more specific prefixes that the downstream uses function quite normally, there is little to no incentive on the part of the downstream to clean this up, unless a large traffic flow to the missing portion causes noticable traffic increase. Stephen
On Wed, Sep 19, 2001 at 12:40:51AM -0700, Ratul Mahajan wrote:
Do you think that it can be a real persistent routing loop (data packets would actually shuttle between the two interfaces) as against some wierdness because of traceroute?
I'd tell you a story, but it would make all of nanog cringe. Put simply, I know of at least one medium size regional network who when they couldn't make "bgp work" decided to implement a series of scripts that logged into all routers and installed statics to make things work. Let me just tell you, after a few short weeks of adding and removing (and missing) such routes there were loops all over the place, many of which are probably still there. :-( There are a number of ways long lived loops can occur. I believe others have pointed out some other methods. -- Leo Bicknell - bicknell@ufp.org Systems Engineer - Internetworking Engineer - CCIE 3440 Read TMBG List - tmbg-list-request@tmbg.org, www.tmbg.org
Do you think that it can be a real persistent routing loop (data packets would actually shuttle between the two interfaces) as against some wierdness because of traceroute?
Seems like a persistent routing loop. One possibility I've heard about is the persistent routing loops that can occur if an internal router is not situated topologically close to its BGP route reflector: http://www.globecom.net/ietf/draft/draft-dube-route-reflection-harmful-00.ht... Does anyone have any idea of if/how often these types of persistent loops occur in practice? I guess there would be no way to really tell if this was the cause without having some knowledge about the internal topology... Nick
participants (5)
-
Leo Bicknell -
Nick Feamster -
Patrick W. Gilmore -
Ratul Mahajan -
Stephen Stuart