Category: Routing

MAC addresses in a VSS cluster

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As you might know, creating a VSS on Cisco Catalyst 4500-X switches is pretty easy and there are many many guides with information how to do this. I think THIS guide is one of the best to do this.
However, there is one additional note which is not mentioned on that (and other) blogs if you are planning to use multiple Catalyst 4500-X VSS clusters. This is related to the switch MAC address.
By default, all MAC addresses used by the Catalyst 4500-X VSS cluster is automatically generated and is based on the VSS domain ID. But what does this mean?
If you’re planning to use multiple VSS clusters in the same network and in the same VLAN(s), you’ll end up with duplicate MAC addresses. I’m sure I don’t have to tell you that this is something you don’t want: it brakes a lot of things in your network.
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OTV FHRP filtering on a ASR router

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We configured a OTV DCI in my previous post and it was working as expected and by design. But during testing of all the VLANs I discovered a problem with HSRP over OTV, but only for 1 specific VLAN. The test results:

  • A ping from a host in DC1 in VLAN 10 to the HSRP address gives random drops
  • A ping from a host in DC1 in any VLAN to the HSRP address pings without any problems
  • Shutdown the SVI of VLAN 10 in DC2, A ping from a host in DC1 in VLAN 10 to the HSRP address without any problems
  • VLAN 10 is still disabled in DC2, but a host can ping the HSRP address from DC2 to DC1. This should be impossible because of the FHRP filtering
  • Changing the standby group number (they are the same in DC1 and DC2 to keep the same MAC address) partially solved the problem, but some hosts in DC1 got the HSRP MAC of DC2 in the ARP table. This is not what we want.
  • Moving the SVI from a 6500 switch to a 3750 switch in DC1, none of the above problems

I still have no idea why this problem only exists for VLAN 10, all other VLANs work as expected but I’ve found a good workaround for this in the configuration guide:
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/wan/command/wan-cr-book/wan-m1.html#wp3953249580
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Configuring OTV on a Cisco ASR

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During a project I’ve been working on, we needed to configure OTV on a Cisco ASR. I did write a blog for configuring OTV on a Nexus 7000 before (click here) but the configuration on a Cisco ASR router is a bit different. The used technologies and basic configuration steps are equal, but the syntax is different for a few configuration steps .
Unfortunately, the documentation is not as good as for the Nexus 7000. I’ve found one good configuration guide (here) but this guide isn’t covering all. So, it’s a good reason to write a blog post about the basic OTV configuration on a Cisco ASR router.
For more information about OTV, check this website.
First, the network layout for this OTV network.
OTV Network layout
 
As you can see in the diagram, the ASR routers are back-to-back connected. There is no guideline how to connect these routers, as long as there is IP connectivity between them with multicast capabilities and a MTU of atleast 1542 btyes.
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OSPF part 2

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LSA Types
There are 6 common used (by Cisco) LSA types:

LSA type 1 updates describe the router itself: interfaces (in the area), list of neighbor routers and the router ID (RID). The RID is transmited by a linkstate-ID in this hello packet. The linkstate-ID is equal to the RID.
LSA type 2 updates represent a transit subnet for which a DR is elected. The LSID are the RID of the DR IP’s address on that particular subnet. I’ts only active on subnets with a active DR. For subnets without a DR (like a point-to-point), type 1 packets are enough for creating the topology database. Inside one area, only LSA types 1 and 2 are send out. All the routers can create a topology table with only these type 1 and 2 LSA’s. A “show ip ospf database” lists all the received LSA types.
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OSPF

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OSPF is a link-state routing protoocol which uses factors such as speed or the link’s shortest path to decide which route is the best.
Link state protocol routers maintain a common picture of the network and exchange link informantion during discovery and/or network changes.
OSPF is designed for large and scalable networks, becauses of the following advantages:

  • Convergence speed
    • OSPF sends only routing changes instead of the entire routing table. Because of the small routing changes, updates are flooded rapidly across the network
  • Support for VLSM
  • Network size
  • Use of bandwidth
    • OSPF uses multicast to advertertise the updates: LSU’s (link state updates) are small in packetsize
  • path selection
    • OSPF selects optimal routes using cost instead of hopcount (RIP)
  • Member groupings
    • OSPF uses areas: every network segment is cut into smaller areas of routers: less LSU’s are sent out and more efficient routing takes place. eh Every router in a area does have the same topology table.

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RIPv2

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RIPv2, isn’t that hard at all.
There are a few enhancements from RIPv1:

  • Authentication: clear text (by default) or MD5
  • Subnet masks: RIPv2 is classless
  • Next-Hop IP address: to avoid routing loops (used in networks with multiple routing protocols)
  • Updates are send by multicast, to IP 224.0.0.9

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