[nsp] Re: strange (dcef?) routing

[jlewis at lewis.org]

This apparently turned out to be caused by having a pair of default routes 
on the 7513 pointing to MFN and Level3.  Remove the static defaults, and 
all traffic for the peer uses the peering connection.  Re-add them, 
routing for some sources goes via the peering connection, others out 
through the transit pipes.  Remove and re-add with metric 250, and routing 
still depends on the source.  I don't see much reason for keeping those 
static defaults, so they've been removed.

How is it that a static default would take precedence over a more specific 
(BGP) route, but only for some source addresses?

Is the hamster in our router smoking crack?

On Wed, 15 Oct 2003 jlewis at lewis.org wrote:

> I just turned up a peering connection between Atlantic.Net and a 
> small network that's multihomed to Sprint, C&W, and Broadwing.  I'm very 
> confused by routing decisions being made on a 7513 that has 2 of our own 
> transit connections and which handles traffic for a bunch of our core 
> network.
> 
> 208.152.224.0/24 is one of the routes received from the new peer.
> 
> On the 7513:
> #sh ip bg 208.152.224.0
> BGP routing table entry for 208.152.224.0/24, version 18173843
> Paths: (3 available, best #2, table Default-IP-Routing-Table)
>   Advertised to non peer-group peers:
>   209.208.6.225 209.208.6.242 209.208.6.250
>   6461 3561 21921
>     209.249.254.146 from 209.249.254.146 (209.249.254.146)
>       Origin IGP, metric 0, localpref 100, valid, external
>       Community: 6461:5997
>   21921
>     69.28.66.46 (metric 20) from 209.208.6.243 (209.208.6.243)
>       Origin IGP, localpref 120, valid, internal, best
>       Originator: 209.208.6.235, Cluster list: 209.208.6.243
>   3356 1239 21921
>     64.156.210.13 from 64.156.210.13 (209.247.3.131)
>       Origin IGP, metric 0, localpref 100, valid, external
>       Community: 3356:3 3356:86 3356:575 3356:666 3356:2006
> 
> #sh ip ro 208.152.224.0
> Routing entry for 208.152.224.0/24
>   Known via "bgp 6364", distance 200, metric 0
>   Tag 21921, type internal
>   Last update from 69.28.66.46 00:58:13 ago
>   Routing Descriptor Blocks:
>   * 69.28.66.46, from 209.208.6.243, 00:58:13 ago
>       Route metric is 0, traffic share count is 1
>       AS Hops 1
> 
> This shows the peering path is the best path.  However, when I traceroute,
> depending on where on our network I do the trace from and which of the
> peer's networks I trace too (all are localpref 120, best via the peering
> connection), the 7513 may send the packets out to Level3, MFN, or use the
> internal path several hops across our network to the peering connection.
> 
> traceroute to 208.152.224.1 (208.152.224.1), 30 hops max, 38 byte packets
>  1  border1-fast0-0.Gainesville.atlantic.net (209.208.0.1)  2.095 ms
>  2  gsvlflma-br-1-s0-0 (209.208.6.125)  1.449 ms
>  3  progress-br-1-s2-0 (209.208.112.133)  4.688 ms
>  4  andc-br-2 (69.28.68.3)  6.160 ms
>  5  gigabitethernet5-1-111.hsipaccess1.Orlando1.Level3.net (64.156.210.13)  6.792 ms
>  6  ge-6-2-0.mp1.Orlando1.Level3.net (209.247.11.37)  7.018 ms00
>  ...
> 
> traceroute to 208.152.224.1 (208.152.224.1), 30 hops max, 38 byte packets
>  1  209.208.127.13 (209.208.127.13)  0.484 ms
>  2  andc-vl-1-gw (69.28.65.1)  0.663 ms
>  3  orldflma-br-2 (69.28.68.8)  2.103 ms
>  4  miamflbr-br-1-s1-1 (209.208.12.186)  11.475 ms
>  5  gw.mmaero.com (208.152.224.1)  14.656 ms
> 
> andc-br-2 and andc-vl-1-gw are different names/IPs/interfaces for the same
> 7513.  Other than CEF bugs, why would the router send packets anywhere
> other than the best BGP path?  The only policy routing currently setup on
> this router is on our transit (MFN and Level3) interfaces blocking
> incoming nachi pings.  The 7513 is running 12.2(14)S1 with dcef enabled.
> 
> ----------------------------------------------------------------------
>  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_________
> 
> 

----------------------------------------------------------------------
 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_________