[c-nsp] Weird Routing Behaviour

[Tomas Daniska]

> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-
> bounces at puck.nether.net] On Behalf Of Lin Domingo
> Sent: Friday, August 13, 2010 5:47 AM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] Weird Routing Behaviour
> 
> I am having weird routing issues with the T1 circuits. We use /30
> subnets for all T1 circuits, so for customer X with assigned subnet of
> x.x.x.176/30, x.x.x.177 is the serial interface on the ISP end of the
> circuit. The ISP side of the router is defined thusly:
> 
> interface Serial6/0/16:0
>  description  XYZ Customer
>  ip address x.x.x.177  255.255.255.252
>  no ip redirects
>  no ip unreachables
>  no ip proxy-arp
>  encapsulation ppp
>  no cdp enable
> 
> 
> Now, I am noticing some weird behaviour:
> 
> (a.) For some reason, the router can not ping (its own) ip  -x.x.x.177 -
> configured on the serial interface!
> (b.) Same router can ping the CPE end of the circuit, in this case,
> x.x.x.178. '
> (c.) Also, if the circuit is bonded multilink T1, the router can ping
> its own ip, but even then, a traceroute to that IP shows that the router
> travels to its own IP by way of the CPE ip!! When pinging its own ip,
> the response time is about 4ms!  When pinging that same ip from a
> neigbhouring router, response time is 1ms!
 
It is my understanding that with some encapsulations, especially point-to-point, the router can reach its own IP only by sending the packet out the link and having it returned by the far end. I don't recall exactly if this applies to ppp or not, and this might be version-dependent as well.

 
> Now, if I do a "show ip route  ip address x.x.x.177", I get:
> 
> Routing entry for x.x.x.128/25
>   Known via "static", distance 1, metric 0 (connected)
>   Redistributing via eigrp 12125
>   Advertised by eigrp 12125
>   Routing Descriptor Blocks:
>   * directly connected, via Null0
>       Route metric is 0, traffic share count is 1
> 
> 
> 
> Why is the route showing as "directly connected, via Null0"  instead of
> "* directly connected, via Serial6/0/16:0"  ? Again if the circuit is a
> bonded multi-link t1, the route shows up correctly as "* directly
> connected, via Multilink10"
> 
> 
> Am I missing something? Someone help please!

Are you sure your Serial6/0/16:0 is up? Seems you have configured a summary route using static to Null0, and you maybe didn't notice the /30 serial is up, lineprotocol down, thus not installing the connected subnet? The other end could advertise its IP (.178) somehow via IGP, maybe as /32, and that might be why you can ping it (it's reachable via other path), but not your own IP (interface is down).
 

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deejay