BGP prefix filter list

Sat May 18 08:57:00 UTC 2019

On Fri, May 17, 2019 at 10:43 PM Blake Hudson <blake at ispn.net> wrote:

> I manage a network like you describe: Two BGP edge routers, both routers
> accept a full eBGP feed from transit, both share routing information via
> iBGP. Both edge routers in my network have a complete view. If one transit
> provider is down or there is an upstream peering change, both still have a
> complete view. The only time they wouldn't have a complete view is during
> convergence or when there is a simultaneous outage of both transit
> providers at different physical facilities.
>
>
What I mean by not having a complete view, is that your two routers do not
have the same information. One router has all the routes from the transit
directly connected, but only a subset of routes from the other transit
provider. And visa versa for the other router. Therefore the two routers
might not make the same routing decisions.

Let me show you an example from two routers in our network:

albertslund-edge1#show bgp vpnv4 unicast vrf internet detail 8.8.8.0
255.255.255.0
BGP routing table entry for 8.8.8.0/24
20w0d received from 193.239.117.141 (66.249.94.118), path-id 0
   Origin i, nexthop 193.239.117.141, metric 100, localpref 500,weight 0,
rtpref 200, best, block best, selected,
   Community 60876:34307
   As path [15169]
   As4 path
   Received label  notag

Imported from 185.24.168.254 (185.24.168.254); Route Distinguisher:60876:0
(default for vrf internet)
   Origin i, nexthop 185.24.168.254, metric 100, localpref 500,weight 0,
rtpref 200,
   Community 60876:34307
   As path [15169]
   As4 path
   Route target:60876:0
   Received label  164540

---

ballerup-edge1#show bgp vpnv4 unicast vrf internet detail 8.8.8.0
255.255.255.0
BGP routing table entry for 8.8.8.0/24
43w1d received from 193.239.117.141 (66.249.94.118), path-id 0
   Origin i, nexthop 193.239.117.141, metric 100, localpref 500,weight 0,
rtpref 200, best, block best, selected,
   Community 60876:34307
   As path [15169]
   As4 path
   Received label  notag

Imported from 185.24.171.254 (185.24.171.254); Route Distinguisher:60876:0
(default for vrf internet)
   Origin i, nexthop 185.24.171.254, metric 100, localpref 500,weight 0,
rtpref 200,
   Community 60876:34307
   As path [15169]
   As4 path
   Route target:60876:0
   Received label  164140

29w2d received from 216.66.83.101 (216.218.252.202), path-id 0
   Origin i, nexthop 216.66.83.101, metric 100, localpref 450,weight 0,
rtpref 200,
   Community 60876:6939
   As path [6939 15169]
   As4 path
   Received label  notag

43w2d received from 149.6.137.57 (154.26.32.142), path-id 0
   Origin i, nexthop 149.6.137.57, metric 200, localpref 100,weight 0,
rtpref 200,
   Community 174:21100 174:22010 60876:174
   As path [174 6453 15169]
   As4 path
   Received label  notag

---

One router knows about 2 paths, the other about 4 paths. Why? Because BGP
only advertises the route that is in use. Everyone here of course knows
this, I am just pointing it out because culling information before allowing
it to be redistributed within your network is what BGP is already doing
anyway. It is possible to remove some of that information from the local
FIB too without losing anything at all.

Using a default also gives you a dramatically shorter convergence time if
one of the transits goes down. Having 800k routes can be harmful to your
network even with equipment that can handle it. Yes I am aware that I am
not doing what I am preaching here, but I am considering it :-).

Regards

Baldur
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