[[nsp] more confused routing]

[joshua sahala]

jlewis at lewis.org wrote:
[cut]
> jcvmfl-br-1#
> Tracing the route to gw.mmaero.com (208.152.224.1)
>  
>   1 jcvlflcl-br-1-s2-0-8.atlantic.net (209.208.112.153) [MPLS: Label 109 
>   Exp 0] 8 msec
>   2 orldflma-br-2-s2-1.atlantic.net (69.28.66.17) [MPLS: Label 208 Exp 
>   0] 24 msec
>   3 andc-br-2.atlantic.net (69.28.68.3) 88 msec
>   4 orldflma-br-2.atlantic.net (69.28.68.8) [MPLS: Label 1235 Exp 0] 24 
>   msec
>   5 miamflgr-br-1-s1-1.atlantic.net (209.208.12.186) 24 msec
>   6 gw.mmaero.com (208.152.224.1) 36 msec
> 
> jcvlflcl-br-1#traceroute 208.152.224.1
>  
> Type escape sequence to abort.
> Tracing the route to gw.mmaero.com (208.152.224.1)
>  
>   1 miamflgr-br-1.atlantic.net (69.28.66.22) 8 msec 8 msec 4 msec
>   2 gw.mmaero.com (208.152.224.1) [AS 21921] 8 msec 8 msec 8 msec
> 
> The two things I don't understand here are 
> 
> 1) when I trace from jcvmfl-br-1, even though jcvlflcl-br-1 knows a 
> shorter path to 208.152.224.1, while jcvmfl-br-1 is using its default 
> route to pass traffic for 208.152.224.1 to jcvlflcl-br-1, traffic gets 
> routed the wrong way several hops into our network before turning around 
> and taking an alternate (longer) path to the destination.  The same 
> trace done from jcvlflcl-br-1 will take the direct route above.
>
> 2) how can orldflma-br-2 route traffic the wrong direction the first 
> time it sees it (hop 2) and the right way the second time (hop 4)?

my mpls skills are rudimentary, but the first time it hits orldflma-br
there is a different mpls label than the second time, so perhaps the
first time it receives it, mpls tells it to go the other way (see if
the interfaces are configured similarly for 69.28.66.17 & 69.28.66.8)

on the olrdflma-br router:

'sho ip route vrf' and see what each vrf has for it's next hop

'sho mpls forw vrf 208.152.224.1 255.255.255.255 [detail]' make sure that
the vrfs are pointing where you expect them to be
 
> I know I can fix this by cleaning up my IGP, but I don't understand how
> routing table changes on an edge router (jcvmfl-br-1) can effect routing
> decisions on core routers that "know better".

mpls?

> If I turn off synchronization on jcvmfl-br-1 and it accepts the bgp 
> route 208.152.224.0/24 into its routing table, the trace takes the 
> appropriate shorter path.
> 
>   1 jcvlflcl-br-1-s2-0-6.atlantic.net (209.208.112.145) [MPLS: Label 
>   141 Exp 0] 4 msec
>   2 miamflgr-br-1.atlantic.net (69.28.66.22) 8 msec
>   3 gw.mmaero.com (208.152.224.1) [AS 21921] 16 msec

this time jcvlflcl-br gets a different mpls label, so i think it is 
something to do with your mpls config and your igp - but like i said,
my mpls skills are laughable, so i may be barking in the wrong zip code

/joshua
  
> ----------------------------------------------------------------------
>  Jon Lewis *jlewis at lewis.org*|  I route
>  Senior Network Engineer     |  therefore you are
>  Atlantic Net                |  
> _________ http://www.lewis.org/~jlewis/pgp for PGP public key_________
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