Commands: ispf

Looking through OSPF I came across a command I did not know about before. The command I came across was ispf, which stands for incremental shortest path first. The benefits of ispf can best be summed up from Cisco's own page on the topic.

OSPF uses Dijkstra's SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the calculating router itself, then the full SPT is performed.

Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.

Looks like quite a useful command to remember, so I will be adding it to my configs in future.

TCP Timeout on IOS

Greg at Etherealmind has done an interesting post on the TCP Timeout command in IOS. I had read about it before many moons ago but it is one of those commands that I tend to forget about. Anyhow Greg's article is a good reminder so click here to read it.

ip bgp-community new-format

Hmm today I discovered another command I did not really know about and probably should have. The ip bgp-community new-format command. After looking it up on the Cisco website I found the following:

To display BGP communities in the format AA:NN (AS-community number/2-byte number), use the ip bgp-community new-format command in global configuration mode.To re-enable the previous display format for BGP communities (NN:AA), use the no form of this command.

ip bgp-community new-format

no ip bgp-community new-format

Usage Guidelines

RFC 1997, BGP Communities Attribute specifies that a BGP community is made up of two parts that are 2 bytes long. The first part is the autonomous system number and the second part is a 2-byte number. In the most recent version of the RFC, a community is of the form AA:NN. The Cisco default community format is one 32-bit number. The ip bgp-community new-format command changes the community format to AA:NN to conform to RFC 1997.

Examples

The following example upgrades a router that uses the 32-bit number community format to the AA:NN format:

Router(config)# ip bgp-community new-format 

The following example shows how BGP community numbers are displayed when the ip bgp-community new-format command is enabled:

Router# show ip bgp 10.0.0.0

BGP routing table entry for 10.0.0.0/8, version 4

Paths: (2 available, best #2, table Default-IP-Routing-Table)

  Advertised to non peer-group peers:

  10.0.33.35

  35

    10.0.33.35 from 10.0.33.35 (192.168.3.3)

      Origin incomplete, metric 10, localpref 100, valid, external

      Community: 1:1

  Local

    0.0.0.0 from 0.0.0.0 (10.0.33.34)

      Origin incomplete, metric 0, localpref 100, weight 32768, valid, sourced, best 

Service Internal

I took the advice of Stretch from PacketLife and decided to research the various lines of our firm's routers configurations. So what happens first command I research I find it pretty hard to find out what it does. The command I was researching was service internal. After some searching round the web I found that it if you apply it in global configuration mode it causes some previously hidden commands to show up.

If anyone else has any knowledge on this command then please let me know.

Pesky VRF's

VRF's (Virtual Routing and Forwarding for the uninitiated) are mentioned quite briefly in the CCNP ISCW exam cert guide. So they are a virtual router yada yada seems quite simple. However I had never seen them in an actual CLI. So imagine my surprise when I said oh such and such route does not exist in the routing table and my colleague turns around and says yes it does. So I do show ip route 10.10.10.10 and no route shows up and then I do show ip route and it still does not show up. Well I cannot see it and then my colleague says did you check the VRF. Ohps the VRF so I do show ip route vrf abc 10.10.10.10 and there it is as clear as day the route I have been looking for. Doh mental note to self remember about VRF's however I still occasionally forget but I am getting better.

So I think I have the hang of these pesky VRF things and then I need to telnet to a particular switch. Hmm cannot get onto it hmm what's going on here I wonder. I pluck up the courage to ask colleague another stupid question "erm mate I cannot access core switch 1, is there an ACL on there that is blocking me?". Reply comes back that all should be fine, so what is going on here. Once again it turns out to be the pesky little VRF stuff that has got me stumped. I need to telnet 11.1.1.2 /vrf abc ah that works.

Then a few days later I want to do a show ip bgp hmm output not as expected. Right this time I twig it has to be one of those pesky VRF's getting in my way. So let's try doing show ip vrf abc wooh no bgp command. Okay lets try figure this out show ip ? and look through the options hmm nothing there so how about a show ip bgp ? hmm nothing there either. Argh so I am stumped how the heck do you do a show ip bgp when you have VRF's around? Time to prepare colleague for another stupid question by offering to make him a drink. Once drink has been made then I utilize my chance for another stupid question. "Umm so you know these VRF's how do you do a show ip bgp turns out the answer is to do a show ip bgp vpnv4 vrf abc. So I have enough trouble with VRF's and then they go add the vpnv4 command before the vrf command. Guess I will get to that bit when I sort out multi-protocol BGP. Anyhow you will be pleased to know that I am getting on a lot better with VRF's now. However as with anything on this blog if any of you have any tips, hints or advice please feel free to post it. I have plenty to learn so welcome any teaching :-)

The Network Command

The network command seems simple enough does it not? Well actually for years I misunderstood the humble network command not that I knew it at the time. Well see ever since I did my first CCNA all those years ago until fairly recently I always thought you used the network command to specify the networks you wanted to advertise. I was wrong as you will hear quite a few times in the Internetwork Expert CoD's (Class on Demand) the network command is not used to specify the networks you want to advertise. Rather it is used to specify the interfaces you want to participate in a particular routing protocol. Well except in the case of BGP where it is actually used to specify what networks you want to advertise to your BGP peer. However BGP is the exception (there always has to be one of those hey?) and in all other routing protocols it is used to specify what interfaces will participate in the routing protocol. I do not think this was emphasised enough in the CCNA materials I used, although when I get some time I will have a browse through the new CCNA materials and see if they mention this small but in my opinion often over looked fact.