[c-nsp] static route to null0 not visible (BGP)

[Alex Foster]

Please excuse the resend - hadn't ccd the list

 

Hi Oli,

 

Your right the /24 is being learned via IGP (OSPF), the following output
is with the metric on the static routes set to 254.  I actually thought
that the /23 would behave the same given that it is also learned through
IGP - or is this not the case?? Perhaps Im confusing things - this is
first time Ive had to advertise more than one netblock.... so forgive my
stupidityness

 

I guess what I really want to do is to advertise the /23 and /24 out of
both peers (routers) - I know that the metric on the static route needs
to be lower than the iBGP admin distance (for this to work) - but cant
understand how my existing /24 works with metrics of 254 on both routers
(when the /23 is configured exactly the same and part of my IGP
configuration).

 

Router-a#sh ip route 83.245.6.0 255.255.255.0

Routing entry for 83.245.6.0/24

  Known via "static", distance 254, metric 0 (connected)

  Routing Descriptor Blocks:

  * directly connected, via Null0

      Route metric is 0, traffic share count is 1

 

Router-a#sh ip bgp 83.245.6.0 255.255.255.0

BGP routing table entry for 83.245.6.0/24, version 1819

Paths: (2 available, best #2, table Default-IP-Routing-Table)

  Advertised to peer-groups:

     transit-provider

  Advertised to non peer-group peers:

  83.245.6.242

  Local

    83.245.6.242 (metric 2) from 83.245.6.242 (83.245.6.242)

      Origin IGP, metric 0, localpref 100, valid, internal

  Local

    0.0.0.0 from 0.0.0.0 (83.245.6.241)

      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced,

Best

 ---------------------------------------------------------

 

Router-a#sh ip route 88.215.60.0 255.255.254.0

Routing entry for 88.215.60.0/23

  Known via "bgp 31655", distance 200, metric 0, type internal

  Last update from 83.245.6.242 00:20:34 ago

  Routing Descriptor Blocks:

  * 83.245.6.242, from 83.245.6.242, 00:20:34 ago

      Route metric is 0, traffic share count is 1

      AS Hops 0

 

Router-a#sh ip bgp 88.215.60.0 255.255.254.0

BGP routing table entry for 88.215.60.0/23, version 263285

Paths: (1 available, best #1, table Default-IP-Routing-Table)

  Advertised to peer-groups:

     transit-provider

  Local

    83.245.6.242 (metric 2) from 83.245.6.242 (83.245.6.242)

      Origin IGP, metric 0, localpref 100, valid, internal, best

 

-----------------------------------------------------------------

 

 

 

-----Original Message-----
From: Oliver Boehmer (oboehmer) [mailto:oboehmer at cisco.com] 
Sent: 04 July 2006 17:55
To: Alex Foster; cisco-nsp at puck.nether.net
Subject: RE: [c-nsp] static route to null0 not visible (BGP)

 

Alex Foster <> wrote on Tuesday, July 04, 2006 6:39 PM:

 

> I have a couple of 3550s with already existing bgp sessions with some

> upstream providers and with iBGP running between them.  Im

> advertising a /24 at present and now want to advertise a /23 netblock

> in addition.  I 

> use static routes to null0 with metric of 254 for the existing /24 (on

> both routers) but when I add new routes to null0 for the /23, only one

> of my routers installs the route - the other learns the /23 via iBGP.

> If I remove the 254 metric on the offending router (and use the

> default 

> metric), the route is installed and advertised.  When the /23 is

> learned 

> by iBGP it has a metric of 200 so does this mean it is preferred over

> the static route ?? - If so why doesn't the /24 installed with a 254

> metric on both routers also behave this way.

 

Both the /23 and /24 should behave this way, i.e. one of the two routers

install the route as static and advertises it via BGP, the other will

learn the route via iBGP and will remove the floating static as the

static has a higher admin distance (254 vs. 200).

Not sure why your /24 doesn't behave this way, "show ip route 83.245.6.0

255.255.255.0" as well as "show ip bgp 83.245.6.0 255.255.255.0" would

help.. Maybe you learn the /24 via an IGP (OSPF) as well, so the OSPF

route wins over both the iBGP and the static route, so BGP will also

advertise the /24 on this router.

 

      oli

 

 

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