Hunt,
The purpose of routing the summary route to Null0 is because it is a
"shortest match" representation of all of the summarized networks, and
therefore is not an actual network that is assigned to an interface. Let's
use the example you gave, where the following subnets:
172.20.10.0/24
172.20.15.0/30
172.20.15.4/30
are summarized using the route 172.20.0.0/16.
Jeff should really have written his explanation this way:
"this route helps to prevent potential black holes when default AND summary
routes are used." (Note the capital AND, indicating that you are using both
a default route AND summary routes, not either/or.)
The summary 172.20.0.0/16 includes lots of potential subnets that aren't
being summarized. If the routers routing table didn't include a route for
the summary, then when receiving a packet destined for, let's say
172.20.55.5 (just a random address that isn't within the range of one of the
3 routes above, but still within the 172.20.0.0/16 summary), it would look
in the routing table and select the default route (0/0) because there is no
more specific match for the destination address. This is a waste of time
because by advertising the summary network to all of the other routers, you
are telling your EIGRP network that all routes that fall within the bounds
of 172.20.0.0/16 are downstream from this router. Because you are doing
this, there can't possibly be a host 172.20.55.5 upstream (i.e. toward the
default route) from this router. Therefore, any packet addressed to a
destination that isn't specifically addressed by one of the networks being
summarized (i.e. a more specific route), you want to discard it (Null0)
because there is no network or host matching the Destination IP. Let's look
at what would happen to a packet destined for 172.20.55.5 without this route
to Null 0.
Step 1 - routerA receives a packet destined for 172.20.55.5
Step 2 - routerA looks in the routing table for the longest match network to
route the packet to.
Step 3 - There is no specific route to the dest. network, so it sends the
packet to the default route with a next hop of routerB
Step 4 - routerB receives the packet destined for 172.20.55.5
Step 5 - routerB looks in it's routing table and finds a matching network,
which is the summary advertised by routerA
Step 6 - routerB sends the packet to the next-hop router for the
summary-route 172.20.0.0/16, which is (routerA)
Step 7 - See step 1
!!! Routing Loop !!! The packet will bounce back and forth until the TTL
is exceeded, then get dropped.
Let's look at the same packet with the route to Null0 for the summary on the
router that is advertising it (routerA)
Step 1 - routerA receives a packet destined for 172.20.55.5
Step 2 - routerA looks in the routing table for the longest match network to
route the packet to.
Step 3 - routerA finds a longest-match in the routing table - 172.20.0.0/16
Next-hop - Null0
Step 4 - routerA discards the packet.
Does this all make sense? You could get away without this route to null0 on
the advertising router if your routes being summarized covered every address
in the summary range, but as this is not always the case, they include the
route to Null0 to protect valuable resources on the router from being
consumed routing packets that have no destination.
Hope this helps.
Kelly Cobean, CCNP, CCSA, ACSA, MCSE, MCP+I
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Hunt Lee
Sent: Thursday, February 28, 2002 12:20 AM
To: [EMAIL PROTECTED]
Subject: EIGRP Question [7:36770]
Hi all,
I have an EIGRP question. It would be greatly appreciated if someone can
shed some light on this.
I found the following Routing Table from TCP / IP Vol1 by Jeff Doyle. But I
don't understand why a summary route would be pointing to Null0?
Jeff explains it as "this route helps to prevent potential black holes when
default and summary routes are used"... which confuses me even more :(
Show ip route
D 192.168.16.0/24 [90/3219456] via 172.20.15.5, 00:41:41, Serial 0
C 192.168.17.0/24 is directly connected, Ethernet 0
C 192.168.18.0/24 is variably subnetted, 2 subnets, 2 masks
D EX 172.25.0.0/16 [170/2221056] via 172.20.15.5, 00:41:48, Serial 0
172.20.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 172.20.10.0/24 [90/2195456] via 172.20.15.5, 00:41:48, Serial 0
C 172.20.15.4/30 is directly connected, Serial 0
D 172.20.15.0/30 [90/2681856] via 172.20.15.5, 00:41:48, Serial 0
D 172.20.0.0/16 is a summary, 00:00:09, Null0
Please help...
Best Regards,
Hunt Lee
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=36840&t=36770
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
