NMC DOiT Vol.2 Scenario 19 – Multicast RPF Checks + Sanity Check Frame Relay Maps + RIP Conditional Default Origination + ip drp server + WCCP


I was not into this lab at all today. It wasn’t the lab itself…I just couldn’t get motivated. I completed the whole thing – it was pretty short. In fact, it was one of the easier labs in the NetMasterClass.com DOiT set, not too much to configure, not too much in the way of mind-bending screwball configuration challenges. I just wasn’t up for it today. I spent just a tad over 7 hours on it, and that included a LOT of dubbing around, experimenting with different techniques (especially in BGP), just to see what would happen, that kind of stuff. After I was done, I went out into the cold (about 18 degrees out there this afternoon, with the wind blowing), and worked on a long-term yard clean-up project. I had a bunch of large-ish pines taken down about a year ago by a friend of mine. He knocked ’em down and bucked ’em up. I’ve been working on hauling the pieces out of the woods and splitting them. I’m a regular Grizzly Adams. :) Today, I didn’t split anything – I just hauled big stumps and stacked them. I don’t want all that wood to lie fallow for another season – it’ll rot, and then it’s no good to anyone. I’ve been pulling as much out of the woods as I can before the snow decides it’s going to stick around for the season. After the scenario, I just needed a little manual labor to clear my head, so I went out and worked.

I finally have a hold on the concept of multicast reverse path forwarding (RPF) checks. When a router receives a multicast stream, he needs to decide if it’s okay to forward copies of that stream out his multicast-enabled interfaces, the concern being loop avoidance. You don’t want multicast packets going round and round the network – that’s just not good. So how does a multicast router decide what multicast streams to forward, and what not to forward? By doing an RPF check. To do an RPF check, the router notes the source IP of the multicast stream, and then checks the unicast routing table for that source IP. Did the multicast stream with that particular source IP arrive via the interface that the unicast routing table points back to for that source IP? Then the multicast stream is good – the router will forward copies out all multicast-enabled interfaces that should get a copy of the stream. Which interfaces those are will depend on whether the routers are running sparse or dense mode, and whether a host downstream from a particular interface has joined the multicast group. Did the RPF check indicate that the multicast stream arrived on an interface other than the one you’d expect based on the unicast routing table? Then don’t forward that stream.

So what other technical joys did this scenario bring?

  • I must admit to being lazy about doing my “show frame-relay pvc” and “show frame-relay map” after I get the frame-relay configured. Today, I decided that I needed to make that a part of my sanity checking. I was hugely surprised to discover a router that, despite being configured correctly with “no frame-relay inverse-arp“, had a couple of dynamic maps across DLCIs I didn’t assign. A reload took care of the problem. That would have been a bonehead-easy thing to overlook on the actual lab. I would have died to have a “78” on my actual lab with 3 points taken off for a problem with frame-relay inverse-arp, of all things. So okay, new sanity check added to my permanent repertoire.
  • I have modified my lab strategy to do IPv6 addressing at the same time I’m doing my IPv4 addressing. My logic is this: I have to craft a lengthy text document with all of the router interfaces and switch VLAN assignments. If I just add the IPv6 link-local and site-local addressing to those interface paragraphs I’m already staring at in my text document, I’m saving time.
  • RIP “default-information originate” has a route-map option to conditionally advertise the default route. In other words, RIP won’t advertise the default route unless the conditions of the route-map are satisfied. In this example, the RIP router will not originate a default route unless is in the routing table.

    access-list 10 permit
    route-map QUADZEROIF permit 10
    match ip address 10
    set interface FastEthernet0/0
    router rip
    default-information originate route-map QUADZEROIF

  • I still haven’t wrapped my brain around it fully yet, but it appears that you need to disable EIGRP split-horizon on interfaces where you have a secondary IP address, if you want that secondary IP prefix to be advertised out that interface. I haven’t sorted out why that is yet, or even if I’m understanding the issue properly. Anyone else been there that can comment?
  • Totally Obscure IOS Feature Of The Day – you can set a router to send metrics to a mysterious Cisco device called a Distributed Director. This would be done with the “ip drp server” command. I didn’t get into reading about this too much, as it appears to a be a highly specialized IOS command group supporting a unique Cisco platform. More information about the Cisco Distributed Director is buried in the Network Management section of the IOS documentation.
  • I was asked to configure a Cat3550 to “offload HTTP requests from the web server”. Logically, this is a web caching function, as that’s what web caches do – they take some of the load off of the web server. I was a little thrown, thinking this function might be a function unique to the 3550’s, but all they were looking for was a basic WCCP configuration which would work on any IOS box. Read more about WCCP here. (1) Enable WCCP globally with “ip wccp web-cache”. (2) Configure an interface to redirect HTTP requests to a web cache, “ip wccp web-cache redirect in”.

    CAT2(config)#ip wccp web-cache
    CAT2#show ip wccp web-cache
    Global WCCP information:
    Router information:
    Router Identifier: -not yet determined-
    Protocol Version: 2.0

    Service Identifier: web-cache
    Number of Service Group Clients: 0
    Number of Service Group Routers: 0
    Total Packets s/w Redirected: 0
    Redirect access-list: -none-
    Total Packets Denied Redirect: 0
    Total Packets Unassigned: 0
    Group access-list: -none-
    Total Messages Denied to Group: 0
    Total Authentication failures: 0
    Total Bypassed Packets Received: 0

    CAT2#show run interf vl20
    Building configuration…

    Current configuration : 94 bytes
    interface Vlan20
    ip address
    ip wccp web-cache redirect in

More in the next post, probably tomorrow. I’m think I’m knocking off for tonight. I have more I want to blog about the BGP scenario, reserving bandwidth with RSVP, and some really interesting layer 2 filtering on the 3550.

About the author

Ethan Banks

Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.

1 comment

  • There is huge number of posts about WCCP in groupstudy.

    As I understood main concepts of WCCP are:
    1. Does not forged enable WCCP globally with the command ‘ip wccp web-cache’
    2. If you think about direction of request, you must think as router: IN – this is incoming packet to the router OUT – this is outgoing packet from the router
    3. ‘ip wccp web-cache redirect in’ usually used when users located within only one subnet, which connected to the router
    4. ‘ip wccp web-cache redirect out’ usually used, when users located on multiple subnets, which connected to the router. This command must be applied to ISP connected interface.
    5. Location is WCCP server is unimportant for WCCP commands
    6. WCCP server can be located in the same subnet as users. ‘ip route-cache same-interface’ command are used to decrease utilization of router, when WCCP server and users located in the same subnet. But this is only performance improvement command. WCCP redirection works without it.


Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.

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