AI, ML, Development + Cisco Learning Blog Learning about Machine Learning, Artificial Intelligence, related devlopment topics and formerly Routing and Switching, Datacenter, Security and other topics, CCIE #23664, Frank Wagner

26. Mai 2006

a simple trunk link an a switch with two vlan’s

Filed under: Bridging + Switching — ocsic @ 14:39

If you want to route between vlan’s you should configure a trunk port for vlan’s to reach each other. If the switch has a routerprocessor, this would not be necessary. But as, for example a 2950 does not have this, you have to configure a trunk.

On the switch you define a port for the trunk

enable
conf terminal
interface fa0/1
switchport mode trunk

That’s all on the Switch side for the trunk port.

Configuring the vlan’s now.

You should add some ports to themenable

configure terminal
interface range fa0/2 – 4 2
interface range fa0/5 – 6 3

So you have vlan 2 and 3 then and also have added both two ports.

Well let’s go to the router now. Check if cabeling is ok. You need a straight-through cable for the connection from the router to the switchport. Well, when this is done, it’s now the turn for the router.

enable
configure terminal
interface e0/0
no shut
interface e0/0.2
encapsulation dot1Q 2
ip add 192.168.2.1 255.255.255.0
no shutdown
interface e0/0.3
encapsulation dot1Q 3
ip add 192.168.3.1 255.255.255.0
no shutdown

These IP Addresses are now the gateway for the switch. Because for example the 2950 Catalyst Switches do not have a router Processor. You can give a VLAN an IP aderesse, but only for management porposes. The management vlan is a special vlan. You can only give the management vlan an ip address. If you try to give more than one VLAN an ip address, you will find out, that every time you want to enable the vlan with „no shutdown“, one VLAN comes up, while the other will go down. This is not possible to have more than one management vlan up.
This is because the 2950 is only a layer 2 Switch. With the 3550 you can do this, but not with the 2950. So for the 2950 you will alway need a router, or so called „on-stick“ configuration, to route between the vlan’s.

Remember also, frames from the native VLAN are not tagged on a trunk, whereas frames from all other VLAN’s are.

Source:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_5_2/cofigide/e_trunk.htm

%CDP-4-DUPLEX_MISMATCH

Filed under: Bridging + Switching — ocsic @ 10:33

This does mean, there is an Duplex Mismatch. Try to configure the port with a static duplex setting. Turn of autosensing. In my experience autosensing very often leads to problems with links. I had this on Gigabyte Ports. The port could not resolve this on his own, i had to setup a static setting to keep the port in a stable state.

You can turn of this messages also with:

no cdp log mismatch duplex

on a router or

turn of cdp on a switch with

no cdp run

either on the interface or globaly.

This could be necessary if you have a dumb device in between. For example connecting a no name switch with a cisco switch. Then the CDP does not discover the speed of the connecting device in the right way and complains about. You should check interface stats, to see if there are any errors on the link. If not, this is an cdp „error“

traceroute and icmp rate-limit unreachable

Filed under: IP and IOS Features — ocsic @ 06:33

I had this problem, that every second packet of a traceroute from a cisco router to another was lost. I could reproduce this easyly. It doesn’t matter, how many other router’s had been in between. So i thought this might be an IOS bug. But i found out it isn’t. It’s a feature. 🙂 As you can see, the second packet is always missing.

Here an example:

r1#traceroute 192.168.2.1

Type escape sequence to abort.
Tracing the route to 192.168.2.1

1 172.20.44.2 16 msec 16 msec 16 msec
2 172.10.144.2 16 msec * 16 msec

r1#traceroute 192.168.2.1

Type escape sequence to abort.
Tracing the route to 192.168.2.1

1 172.20.44.2 16 msec 16 msec 16 msec
2 172.10.144.2 20 msec * 16 msec

To limit the rate of ICMP unreachable, there is an limit to 500 ms. This is a protection for denial of service attacks.

If you want to turn it of, you can use the

“ no ip icmp rate-limit unreachable“ command.

There is a quite good description of the traceroute command on the ciscowbsite. Cisco site info on traceroute

I found this quite interesting.

23. Mai 2006

Automatic Route Summarisation

Filed under: IGP Routing — ocsic @ 18:23

The following Routing Protocols do automatic route summarisation by default:

  • EIGRP
  • IGRP
  • RIP v1

OSPF Route Types

Filed under: OSPF — ocsic @ 17:54

In the routing table you can see descriptions about the entries.

Here are some more explanations.

  • O – OSPF
  • IO – OSPF inter area
  • N1 – OSPF NSSA external type 1
  • N2 – OSPF NSSA external type 2
  • E1 – OSPF external type 1
  • E2 – OSPF external type 2

OSPF LSA Types

Filed under: OSPF — ocsic @ 17:51

Also important to know are the LSA Types OSPF speaks to other routers

The type indicates and tells something about the area type. The following exist:

  • Type 1  is an info about routers and directly connected links
  • Type 2 info about LAN and routers connected
  • Type 3 Networks reachable outside the area
  • Type 4 reachability info to the autonomous system board from the ABR
  • Type 5 info about destinations outside the OSPF domain from an ASBR
  • Type 6 multicast info
  • Type 7 NSSA area

OSPF Areas

Filed under: OSPF — ocsic @ 17:47

When you start with OSPF, everything seems to be simple. But later on you learn about many differnent types of OSPF areas.

  • Stub area – has no connection to external networks, it does not contain any external routes, relieys mostly on default routes, does not have AS-external routes (Type 5 LSAs)
  • Totaly Stub area – is an extension to the stub area and includes external routes and routes to other area over a default route, get these by Typ 3LSAs, does not support summary routes
  • Backbone area – is always area zero
  • NSSA, Not so stubby area – the same as SA, but can import external routes and send them to the backbone or other areas, type 3 and type 4 summary routes are not advertised into NSSA

Source:

http://en.wikipedia.org/wiki/Open_Shortest_Path_First

ospf – summarize areas

Filed under: OSPF — ocsic @ 10:26

If you want to summarize areas in the routing table of router behind an ABR you could use the following command in OSPF Router configuration mode.
area 1 range 192.168.1.0 255.255.255.0

With this command, als subnets belonging to 192.168.1.0 are not showing up in the routing table of routers behind the ABR.

Stub area

Filed under: OSPF — ocsic @ 08:23

If you don’t want to show up all your routes in a routing table of a stub router, use the „no-summary“ command at the router next to the stub router.

If you have one router configured with the „area 1 stub“ command use the „area 1 stub no-summary“ command on the other router to just let a default router show up in the stub router.

19. Mai 2006

Route summarization

Filed under: IGP Routing — ocsic @ 20:31

Routing protocols summarize networks into supernetworks to keep routing tables small.

Rip does this by default and you can’t turn it off.

EIGRP does this also by default. But you can turn it of with „no auto-summary“

And last but not least IGRP does this also by default.

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