[c-nsp] conditional bgp default-originate

[David Freedman]

silly question, but why not ask your provider for a default route in
with your feed and simply just propagate it downstream??

Dave.


Jon Lewis wrote:
> I'd like to be able to conditionally advertise a default route to
> customers taking just default routes only if my transit BGP sessions
> appear to be functional.
> 
> I thought something like this might work:
> 
>  neighbor 10.1.0.2 default-originate route-map BGP-UP
> 
> route-map BGP-UP permit 10
>  match as-path 100
> 
> ip as-path access-list 100 permit ^3356_
> ip as-path access-list 100 permit ^4323_
> 
> But no such luck.  Checking the docs at
> 
> http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_n1g.html#wp1037042
> 
> 
> it seems I have to exactly match against a route for the route-map to
> work here.  That means actually picking a few "canary routes" I expect
> to get from my upstreams and hoping they don't go anywhere or change
> mask.  I'm not really happy with that.  Are there better ways to do this?
> 
> Also, while looking at the docs above and experimenting in the GNS3
> simulator (emulated 2600s running c2600-i-mz.123-26.bin), I've found a
> few oddities.
> 
> First, there's multiple errors in the docs mentioned above.  i.e. From
> the URL above:
> 
>  In the following example, the last line of the configuration has been
>  changed to show the use of an extended access list. The local router
>  injects route 0.0.0.0 to the neighbor 172.16.2.3 only if there is a route
>  to 192.168.0.0 with a mask of 255.255.0.0:
> 
>  router bgp 50000
>   network 172.16.0.0
>   neighbor 172.16.2.3 remote-as 60000
>   neighbor 172.16.2.3 default-originate route-map default-map
>  !
>  route-map default-map 10 permit
>   match ip address 1
>  !
>  access-list 100 permit ip host 192.168.0.0 host 255.255.255.0
> 
> In the above example, they did change the ACL to an extended
> access-list, but the route-map wasn't updated to use it (still using 1)
> and they say they're looking for 192.168.0.0 with a mask of 255.255.0.0,
> but the access-list 100 uses a /24 mask.
> 
> Just above this example, the docs say that
>  access-list 1 permit 192.168.0.0
> will match a route for 192.168.0.0 with any mask.  In my simulator, I
> have R1--R2--R3
> R1 advertises 8.0.0.0/16 to R2.  R2 is advertising a conditional default
> to R3 using the route-map
> 
> route-map BGP-UP permit 10
>  match ip address 50
> 
> access-list 50 permit 8.0.0.0
> 
> When R2 receives 8.0.0.0/16 from R1, there are no hits on the ACL and
> default is not sent ot R3.  If I add to access-list 50
> access-list 50 permit 8.0.0.0 0.0.255.255
> 
> Standard IP access list 50
>     10 permit 8.0.0.0 (973 matches)
>     20 permit 8.0.0.0, wildcard bits 0.0.255.255
> 
> I get hits on the permit 8.0.0.0 line now, and default is sent to R3.
> This seems kind of broken.  I haven't duplicated the setup with real
> hardware to see if it's a simulator screwup...but since the simulator is
> running actual IOS, it seems unlikely the simulator is to blame.
> 
> ----------------------------------------------------------------------
>  Jon Lewis                   |  I route
>  Senior Network Engineer     |  therefore you are
>  Atlantic Net                |
> _________ http://www.lewis.org/~jlewis/pgp for PGP public key_________
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