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Home > Archive > CCNP > July 2001 > null zero
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| buntywins 2001-07-23, 2:01 am |
| hi can anyone explain 2 me the concept of null zero i have read the article in bgp section of cisco press book many times but i still cant figure it out
bunty | |
| Retired-Mod 2001-07-23, 3:28 am |
| A static route to null zero gives a path for packets with an unknown destination to be dropped. It is often referred to as the "bit bucket".
Retired | |
| doctorcisco 2001-07-25, 1:29 pm |
| quote:
Originally posted by buntywins
hi can anyone explain 2 me the concept of null zero i have read the article in bgp section of cisco press book many times but i still cant figure it out
bunty
As retired said, Null0 is something like a loopback interface without an IP address, AKA the bit bucket. Null0 never goes down, so (unless the router itself dies) you'll never stop advertising out your summary route.
When you're summarizing routes in BGP, (or in EIGRP, which also routes summaries to Null0), remember that the summary isn't telling that router where to send the traffic. You're telling the rest of the world, "Send all traffic for this address space to me; I'll take it from there." This makes for much smaller routing tables. The route summary also keeps routing changes inside your AS from causing routing updates throughout the entire internet. One of BGP's main purposes is to prevent local, transient problems within one AS from causing route recalculations everywhere in the world.
When traffic arrives for the addresses you summarized, the BGP router will use its routing table to decide where to send the traffic within your AS. The router originating the summary won't have one router as a next-hop address for all those addresses -- there are most likely several next-hop routers for different pieces of the address space you're summarizing. However, the summary route needs to be in the routing table so that you can advertise it, and you need a next-hop of some kind in a routing table. So ... on the router originating a summary route, in BGP or in EIGRP, the next-hop is Null0. This tells the router, "This is a summary route you're advertising out. Use other routing information to actually forward the traffic. If you have no route to the destination (something went down), send back an ICMP Destination Unreachable and drop the packet. But continue advertising this summary until a human being tells you otherwise."
HTH,
doctorcisco | |
| richard21 2001-07-25, 3:48 pm |
| A great explanation, many thx Doctor Cisco. | |
| Retired-Mod 2001-07-25, 5:12 pm |
| quote:
Originally posted by doctorcisco
When you're summarizing routes in BGP, (or in EIGRP, which also routes summaries to Null0), remember that the summary isn't telling that router where to send the traffic. You're telling the rest of the world, "Send all traffic for this address space to me; I'll take it from there." This makes for much smaller routing tables.
I have seldom contradicted the good Doctor, but this is a bit misleading. When you add a Null 0 static route it will be the least specific route and there for only used to dump traffic destined for an unknown network. This is NOT a route you want any traffic to take, but is set to remove bogus traffic from your network.
The good Dr sounds like he is describing a gateway of last resort more then a Null Zero.
Just me two cents,
Retired | |
| strikeattack 2001-07-28, 8:07 am |
| Here is my perspective...
When summarization is configured on an interface, a summary route is added to the routing table with a reference to Null0, a directly connected, software-only interface. The use of the Null0 interface prevents the router from trying to forward traffic to other routers in search of a better route. This is a virtual interface on the router that goes nowhere.
Because routers choose the MOST-SPECIFIC route available from the routing table, you should never end up actually using the null 0 interface. However, if some interfaces go down, and the summary route is the only route available, the packets will start to end up in the bit bucket. This is conceptually different than simply dropping them, and it is important to know the difference. |
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