At 03:27 PM 4/17/02, Stephane LITKOWSKI wrote: >Hi all, > >I have some questions about IGRP : > >- In an update, IGRP distinguish Subnets, AS Networks and outside-AS >networks, when I take some traces, I can see that there's the number of each >category and then all routing entries but I don't understand how IGRP know >if the routing entry is Subnet, AS or outside AS. Distinguishing AS routes >(major net) or subnets is not important (applying interface mask and >comparing major networks can do distinction), but how IGRP knows if the >routing entry is candidate default ?
An IGRP update message has three portions: interior, system (meaning "this autonomous system" but not interior), and exterior. (Sniffer uses slightly different terminology.) The interior section is for routes to subnets for the network on which the broadcast is being sent. Major networks (i.e. non-subnets) are put into the system portion of the update message unless they are exterior. A network will be flagged as exterior if it was learned from another router and the information arrived in the exterior portion of the update message. Exterior routes are also "candidate defaults." They are routes that go to or through routers that are considered to be appropriate as defaults, to be used when there is no explicit route to a destination. Cisco's implementation chooses a default route by picking the exterior route with the smallest metric. >- I can see in traces that IP addresses are coded with just 3 bytes, so I >think I cannot use subnets > 24 bits ... isn't it ? Not necessarily. Class C subnets could be greater than 24. With interior routes, a router can assume that the first octet is the same as the first octet of the sending and receiving interfaces, so it can be left out. Remember that IGRP is classful, so there's no way the prefix boundary could be over to the left past the 8-bit boundary, so IGRP leaves out the first 8 bits in interior (subnet) routes. Check your traces again. I think you'll see that it's the first octet (not the last) that is left out on interior routes. On system and exterior, which deal with major network numbers, the router can leave out the last octet. >- I can see different routing entry types, for example (traces done with NAI >Sniffer Pro) : > >ROUTING ENTRY #1 > IP Address = [172.16.1.0] > ... > >ROUTING ENTRY #2 > IP Address = [10.0.0.XX] >... > >what does the XX mean ??? I can see in hexa dump that's 00 Which kind was this? System or exterior I would guess. Perhaps the XX just means anything (wildcard). It's probably not actually in the hex dump, since IGRP just uses 24 bits as you mentioned. Check for 0A 00 00, which would be 10.0.0. >- What is the utility of the hopcount ? preventing from loop like in RIP ? Yes, it prevents loops. I think if the router sees the hop count increasing, it puts the route in holddown. >(so is the hop count incremented hop by hop ?) Yes. >- I saw that Cisco implement trigger updates in IGRP, but on a Cisco 1603R >with IOS 12.0(4)T, it don't seem to be implemented, when there's a change in >topology, there's no trigger update. Does anyone experienced this problem ? There should be a triggered update. It will list the unreachable network with the largest possible delay. Could you check again? Perhaps in some topologies, there would be no need to send a triggered update, but that would be strange. I think you maybe just didn't recognize it as being a triggered update. >- Does IGRP use a database like RIP ? or not ? It probably uses something like a database. For some reason, there aren't very good show commands for igrp though. Of course there's show ip route and show ip route igrp, but I don't know of others..... By the way, when trying to figure out IGRP, I go to this old paper here: http://www.cisco.com/warp/public/103/5.html Hope that helps. Priscilla >Can I see the database like >"show ip rip database" ? > > >thanks a lot. > > >Stephane ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=41780&t=41763 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
