Dear Community, I am studying for the CCNP ROUTE exam hoping to pass it before the Feb deadline and had a couple of questions about OSPF operations that I was hoping you could help me out with:   1) How exactly does the router use the received Router LSA to build its OSPF tree and subsequently select the best possible route to put into the RT? I know the LSA includes all kinds of info, but at the end of the day what it really seems to boil down to is the metric right? Since OSPF calculates the cost based on incremented path bandwidth, what is the point of the router knowing about the other fields in the LSA like Link Type or even the RID of the originating router? These fields don't seem to really serve any purpose for path calculation, at least to me they dont.   2) What is the purpose of the Network LSA that the DR advertises? Why do the other routers on the multi-access network need to be explicitly told about what other routers are fully adjacent to the DR? I cant figure why thats important to the other routers.   Thanks for any help you can provide. ... View more

Community, I am studying for the CCNP ROUTE exam and noticed something interesting in the routing table for one of the routers in my GNS3 Lab that I have never noticed before:   When the router didnt have a locally connected 10.0.0.0/8 network it presented the routes for the 10.0.0.0 network like this:   10.0.0.0/24 is subnetted, 4 subnets D 10.9.0.0 [90/409600] via 199.0.0.1, 02:12:18, Ethernet0/1 D 10.10.0.0 [90/409600] via 199.0.0.1, 02:12:18, Ethernet0/1 D 10.11.0.0 [90/409600] via 199.0.0.9, 02:12:11, Ethernet0/2 D 10.12.0.0 [90/409600] via 199.0.0.9, 02:12:11, Ethernet0/2   However, when I add a loopback interface in that major network to that router, the RT now looks like this:   10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks D 10.9.0.0/24 [90/409600] via 199.0.0.1, 02:15:04, Ethernet0/1 D 10.10.0.0/24 [90/409600] via 199.0.0.1, 02:15:04, Ethernet0/1 D 10.11.0.0/24 [90/409600] via 199.0.0.9, 02:14:57, Ethernet0/2 D 10.12.0.0/24 [90/409600] via 199.0.0.9, 02:14:57, Ethernet0/2 C 10.20.0.0/24 is directly connected, Loopback1 L 10.20.0.1/32 is directly connected, Loopback1   Im noticing it now says 10.0.0.0/8 is variably subnetted and now provides the subnet mask with each individual prefix. Where when the router didnt have that subnet locally connected it didnt look like that. Can someone please help me understand this?   Thanks.  ... View more

Community, I am labbing up RIPv2 in GNS3 and came across something interesting. The CCIE R/S v5.0 book states the following about the Holddown timer in RIPv2: "Router starts the Holddown timer for this network. While the timer runs (180 seconds by default), the router itself advertises the network with infinite metric (Route Poisoning) to force its neighbors to find an alternative route if possible. Additionally, the router locks the routing entry in its routing table, still pointing toward the former next hop. Absolutely no updates whatsoever are accepted until the Holddown timer expires, regardless of who sends them and what metric they claim."   This passage implies that the router will advertise the invalidated route with an infinite metric for the duration of the Holddown timer. However, I have found this to not be true in my lab. I have found that the router will continue to advertise the invalidated route as unreachable for (Flush timer - invalid timer). Essentially the router will continue to advertise the route as invalid for the remaining period on the Flush timer and even AFTER the Holddown timer has expired unless it receives an update about the invalidated route between the Holddown timers expiry and the Flush timers expiry. So it would seem the Holddown timer does not really affect how long the router advertises the route as inaccessible. However, if the router does receive a valid update about the inaccessible route after the Holddown expires but before the Flush expires, it will send a flash update regarding that and stop advertising the route as unreachable.   Can anyone corroborate this finding? Thanks!   ... View more

Community, I am currently studying for the CCNP ROUTE exam and am labbing up Frame Relay and RIPv2 in GNS3. I am noticing something interesting in my lab. I have 3 routers connected to a FR switch in the middle. The hub router is using a physical multipoint interface, so Split Horizon is disabled. The spoke routers are using multipoint subinterfaces, so split horizon is enabled. The spoke routers do not have any DLCI mapping to eachother on the FR switch so all communications have to go thru the Hub router.   The issue I am noticing is that when the spoke routers send RIPv2 updates to the Hub, the hub is accepting the updates and entering the subnets into its RT. *Sep 5 21:16:06.238: RIP: received v2 update from 199.0.0.2 on Serial1/0 *Sep 5 21:16:06.238: 192.168.1.0/24 via 0.0.0.0 in 1 hops *Sep 5 21:16:19.880: RIP: received v2 update from 199.0.0.3 on Serial1/0 *Sep 5 21:16:19.880: 172.16.1.0/24 via 0.0.0.0 in 1 hops   With SH disabled on this interface, the Hub router is attempting to advertise the routes it learned back to the routers from which it learned them (this is expected with SH being disabled).   Sep 5 21:27:05.940: RIP: sending v2 update to 224.0.0.9 via Serial1/0 (199.0.0.1) *Sep 5 21:27:05.940: RIP: build update entries *Sep 5 21:27:05.940: 10.10.10.0/24 via 0.0.0.0, metric 1, tag 0 *Sep 5 21:27:05.940: 172.16.1.0/24 via 199.0.0.3, metric 2, tag 0 *Sep 5 21:27:05.940: 192.168.1.0/24 via 199.0.0.2, metric 2, tag 0 *Sep 5 21:27:05.940: 199.0.0.0/29 via 0.0.0.0, metric 1, tag 0   However, when the hub routers who do have SH enabled receive the RIPv2 update from the hub, they are NOT seeing or otherwise acknowledging their own subnets advertised back to them from the Hub:   This router is not seeing or processing 192.168.1.0/24 from the advertisement from the Hub:   Sep 5 21:27:05.965: RIP: received v2 update from 199.0.0.1 on Serial1/0.1 *Sep 5 21:27:05.965: 10.10.10.0/24 via 0.0.0.0 in 1 hops *Sep 5 21:27:05.965: 172.16.1.0/24 via 199.0.0.3 in 2 hops *Sep 5 21:27:05.965: 199.0.0.0/29 via 0.0.0.0 in 1 hops   This router is not seeing or acknowledging 172.16.1.0/24 from the advertisement from the Hub:   *Sep 5 21:27:05.964: RIP: received v2 update from 199.0.0.1 on Serial1/0.1 *Sep 5 21:27:05.964: 10.10.10.0/24 via 0.0.0.0 in 1 hops *Sep 5 21:27:05.964: 192.168.1.0/24 via 199.0.0.2 in 2 hops *Sep 5 21:27:05.964: 199.0.0.0/29 via 0.0.0.0 in 1 hops   Is Split Horizon responsible for this phenomenon? In reading about Split horizon, I am only reading about how SH prevents a route from being advertised on the interface from which it was learned, but I have never seen any literature state the SH would PREVENT the router from processing or acknowledging route entries that the router itself owns.    Any help on this would be greatly appreciated! Thanks.  ... View more

Dear Community, I am currently studying for the CCNP-ROUTE exam in an attempt to pass it before the February 2020 deadline. I am currently studying section 1.3b that has to do with IPv4 and IPv6 fragmentation and I had a couple of questions: 1) When the originating host decides that it needs to fragment the IPv4 packet, how does the host itself know that a downstream MTU is smaller than some of the packet sizes and that the packets need to be fragmented.  2) If an IPv4 packet that is not fragmented needs to be fragmented in transit, or is fragmented already and needs to be fragmented again, can an intermediate router in the path perform the fragmentation? if so, under what circumstances would this occur?   I know these questions may be a little "in the weeds" for the exam but I always try to be as prepared as possible. I appreciate any feedback you can provide regarding the fragmentation process of IPv4.   Thanks! ... View more