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FHRP Filtering on Cisco ASR1001 with OTV

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I’m finally getting the chance to deploy OTV and LISP in a live environment and wanted to share one of the issues I’ve run into.

As I mentioned in my post about OTV Traffic Flow Considerations, using HSRP (or VRRP/GLBP) at each site has the potential to cause traffic to “trombone” through the network in a sub-optimal path. Because of this behavior, FHRP filtering should be configured on your OTV routers to ensure that the HSRP device on each side of the overlay becomes an active gateway for the network. The ASR1001 is supposed to have this built-in.

Here’s the topology:

Production OTV Diagram

The Problem

After I setup OTV and LISP, I noticed that I had spotty connectivity to my host inside the overlay. A continuous ping revealed that I was missing a ping or two almost every 60 seconds. When I looked at the route for that host, the age was always less than 1 minute. Since these routes are redistributed into OSPF, I went back to the OTV/LISP routers and tried to see what was happening.

On the OTV/LISP routers, I could see that the local Lisp routes were also being inserted and withdrawn regularly, which meant that Lisp thought the EID was moving to the other router. Since the LISP mapping system is in charge of communicating EID-to-RLOC mapping changes, I ran debug lisp control-plane map-server and observed the following output (abbreviated):

Oct  2 11:09:41.623 EDT: LISP: Processing received Map-Notify message from 10.5.7.82 to 10.5.8.82
...
Oct  2 11:09:41.623 EDT: LISP-0: Local dynEID MOBILE-VMS IID 0 prefix 10.78.1.245/32, Received map notify (rlocs: 1/1).
Oct  2 11:09:41.623 EDT: LISP-0: Local dynEID MOBILE-VMS IID 0 prefix 10.78.1.245/32, Map-Notify contains new locator 10.5.7.82, dyn-EID moved (rlocs: 1/1).

Since I hadn’t moved the VM across the overlay, It surprised me to see that LISP thought the VM was moving. After banging my head on the wall with that issue, I started looking lower in the stack at OTV.

During normal operation, the OTV routing table on the local OTV router (router closest to the host) should look like this:

SAV-OTVRTR2#sh otv route
...
OTV Unicast MAC Routing Table for Overlay1

Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------
0    800  800    0000.0c07.ac4e 40    BD Eng Gi0/0/0:SI800
...

Note the route for 0000.0c07.ac4e , which is the MAC for HSRP group 78. This is a FHRP address, so should it even be showing up? Since it was there I assumed that the FHRP filtering must only prevent the route from being advertised to OTV neighbors.

But during one of the blips, I noticed this:

 Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------
0    800  800    0000.0c07.ac4e 30    ISIS   RAD-OTVRTR2

So not only was the HSRP MAC showing up with FHRP Filtering enabled, but it was also still being advertised across the network. This shouldn’t be.

The Solution – for now

I opened a TAC case and consulted with Cisco about the issue. They agreed that it was “odd” that the HSRP information was leaking across the overlay and recommended I put in an ACL to block FHRP information:

mac access-list extended otv_filter_fhrp
 deny   0000.0c07.ac00 0000.0000.00ff host 0000.0000.0000
 deny   0000.0c9f.f000 0000.0000.0fff host 0000.0000.0000
 deny   0007.b400.0000 0000.00ff.ffff host 0000.0000.0000
 deny   0000.5e00.0100 0000.0000.00ff host 0000.0000.0000
 permit host 0000.0000.0000 host 0000.0000.0000

…and apply the ACL to the OTV Inside interface.

You might notice that OTV automatically adds another ACL:

Extended IP access list otv_fhrp_filter_acl
    10 deny udp any any eq 1985 3222 (57416 matches)
    20 deny 112 any any
    30 permit ip any any (51921 matches)

This ACL blocks the UDP ports used for HSRP and GLBP, as well as IP Protocol 112, VRRP. This must be the portion that is added by default, but it doesn’t seem to be sufficient.

Conclusion

I asked Cisco about why the extra ACL was necessary when the documentation indicates that FHRP was built-in and enabled by default. As soon as I hear something I’ll provide an update. As far as I know the Nexus 7K still requires you to manually configure these ACL’s, but it seems that, for now, so do the ASR’s.

 

Update: I heard back from Cisco TAC about my issue and they think my problem stems from the fact that I’m trying to use the same physical hardware for both the L2 bridging and the L3 gateway:

Due to the ASR1k architecture, it is recommended that you move FHRP off the ASR. It is unlike N7k architecture where we can keep FHRP on the same device and use a mix of MACLs, VACLs, etc to filter out the virtual MAC from going across the overlay. The only way to really prevent the virtual MAC from being learned across the overlay is to prevent the ASR from ever learning it in the first place.

In regards to the default OTV FHRP filtering, TAC confirmed that the otv_fhrp_filter_acl is added when OTV is configured.  It doesn’t attempt to prevent L2 information from being learned however — it only attempts to block actual HSRP communication across the overlay.

Cisco LISP

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With technologies like OTV, we need a method to optimize traffic destined for our mobile virtualized hosts, by tracking the location and updating our underlying routing system. One possible solution is known as LISP.

LISP stands for Locator ID Separation Protocol, and its function is to allow you to separate the location component of an IP address, from the identity portion of the address. Or as the Cisco LISP configuration guide states, LISP “implements the use of two namespaces instead of a single IP address.” What does this mean?

With LISP you are introduced to two new concepts:

  • Endpoint Identifiers (EID)
  • Routing Locators (RLOC)

The endpoint identifier (EID) is the address used to identify a specific host — this is the same as the IP addresses you use today, and it is said to be in the LISP namespace. The Routing Locator (RLOC) is the address of a router that is part of the normal routing domain but is connected to the LISP namespace and the non-LISP namespace. The RLOC is said to be part of the non-LISP namespace.

One significant difference with LISP is that you no longer have to advertise the EID address space into the normal routing domain. Instead, you rely on LISP to provide mappings between EIDs and RLOCs, and you route based on the RLOC address.

I like to think of LISP as a DNS-like system for routing, because an important part of LISP is a mapping system that maintains EID-to-RLOC mappings. Just like you use DNS to query for name-to-IP mappings, a LISP router performs a query against a LISP mapping server to find out the RLOC that should be used to reach the desired EID.

Components of a LISP system

An complete LISP infrastructure will consist of many parts:

  • Ingress Tunnel Routers (ITR)
  • Egress Tunnel Routers (ETR)
  • XTR
  • Map Server (MS)
  • Map Resolver (MR)
  • Proxy Ingress Tunnel Routers (PITR)
  • Proxy Egress Tunnel Routers (PETR)
  • PXTR

ITR

The ingress tunnel router receives unencapsulated IP packets from the EID namespace, and is responsible for performing lookups to identify EID-to-RLOC mappings for destination addresses. If the packet is destined for an EID in another LISP namespace, the ITR will encapsulate each packet with a LISP header and route the packet towards the identified RLOC. If the packet is destined for a non-LISP address, the packet is routed without any LISP modifications.

ETR

The egress tunnel router receives LISP encapsulated packets from the non-LISP portion of the network, removes the LISP header, and delivers the unencapsulated packets to the EID. The ETR is also responsible for keeping the Mapping system up to date with EID mappings and responding to Mapping system requests.

LISP XTR

An XTR is a router that performs both ETR and ITR functions.

Map Server

The map server receives EID registrations from ETRs, and responds to map request messages that are forwarded from map resolvers.

Map Resolver

The map resolver receives encapsulated map request messages from ITRs and forwards them to Map Servers that are authoritative for the EID namespace being queried.

PITR/PETR/PXTR

The Proxy ITR/Proxy ETR allows non-LISP sites to communicate with LISP sites, and vice-a-versa, by performing ITR and ETR functionality. They can be deployed separately, or together. If deployed together the device is referred to as a PXTR.

Conclusion

It took me a little while to wrap my head around the LISP concept, and one big help was working with it in a lab environment.  In the next post, I’ll walk through a lab scenario to demonstrate LISP in action.

OTV Traffic Flow Considerations

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The beauty of OTV is that you are no longer limited to segregating your L2 VLAN’s based on site, or location within your network.  When used in conjunction with Virtual Machines, this means you can migrate machines between locations without having to modify IP addressing, giving you the ability to move entire server farms with only a few clicks.

Beauty has an ugly side, however. One of the not insignificant challenges with OTV is knowing how to best reach endpoints within the overlay network. Improper planning in this area can result in inefficient traffic flows through your network, and could possibly block end-end traffic altogether. Consider the following network:

lisp-network

Let’s say your host is in DC1, but your gateway is in DC2. How will traffic move through the network?  Often called ‘traffic tromboning,’ this is where traffic enters through one side of your network, and uses the overlay to trombone across to the opposite side before returning back through the original datacenter.

lisp-network-trombone

It’s ugly, but you can fix that by using an FHRP to have a gateway in each site. As we know, the ASR’s have FHRP filtering configured and enabled by default, and there is documentation on how to configure filtering for the N7K. After adding gateways to both sites, you end up with this:

lisp-network-trombone2

Well, that might be ok — if you turn a blind eye to the traffic flowing across  your core multiple times, but it’s certainly not the most efficient.  But to add insult to injury, what if your two sites have their own path out to the internet?  How will your edge firewall respond when it receives traffic for a connection it doesn’t know about?

Conclusion

These are just some of the issues that need to be considered when evaluating an OTV solution.  Multiple entry/exit points, firewall placement, flow lifetime, load-balancers, etc. combine to make the overall design complicated very quickly.   Add  in endpoint mobility (the whole point, right?), and you have to ensure that new flows will know how to reach the correct endpoint, and old flows either persist or can be reestablished quickly.  In my next post, I’ll discuss one of the solutions I’m exploring to solve these issues.

Cisco OTV – Overlay Transport Virtualization – Part 2

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In part 1 we configured OTV using multicast as the control plane transport method. But if you don’t want to use multicast, you can use unicast instead. The main difference is that you’ll also need to designate at least one OTV Edge device to act as an Adjacency Server.

Adjacency Server

With OTV in multicast mode, the underlying multicast infrastructure handled packet replication to each of the remote OTV devices, and also allowed the dynamic discovery of OTV peers. With unicast mode, each edge device must send a unicast copy of the packet to each remote device. The question is – how do they know which devices should receive each packet?

The adjacency server function allows a router to learn of, and distribute lists of edge devices, so all members of the OTV domain are aware of each other. The adjacency server creates a Unicast Replication List (URL) and distributes this list to each edge device, updating as necessary when devices join or leave.

Once you have your adjacency servers configured, adding a new site is as easy as configuring the new device with the address of the two Adjacency servers, who will then distribute information about the new device to the rest of the overlay.

Configuration

Since OTV can use only one mode at at time (unicast OR multicast) you must completely remove any multicast specific OTV commands before you can add the adjacency server config.

So we’ll start by removing our multicast control-group information from both routers:

interface Overlay1
no otv control-group 239.1.1.1
no otv data-group 232.1.1.0/28

Since our topology only has two routers, we’ll only configure OTV-RTR1 to act as the adjacency server, and then point it to itself:

interface Overlay1
otv adjacency-server unicast-only
otv use-adjacency-server 10.80.0.2 unicast-only

On OTV-RTR2, we’ll only specify the adjacency server address:

interface Overlay1
otv use-adjacency-server 10.80.0.2 unicast-only

Now let’s look at the status of the overlay:

OTV-RTR1#sh otv overlay1
Overlay Interface Overlay1
 VPN name                 : None
 VPN ID                   : 1
 State                    : UP
 AED Capable              : Yes
 Join interface(s)        : GigabitEthernet0/0/1
 Join IPv4 address        : 10.80.0.2
 Tunnel interface(s)      : Tunnel1
 Encapsulation format     : GRE/IPv4
 Site Bridge-Domain       : 100
 Capability               : Unicast-only
 Is Adjacency Server      : Yes
 Adj Server Configured    : Yes
 Prim/Sec Adj Svr(s)      : 10.80.0.2

We can see that this router is an adjacency server, and that it has an adjacency server configured. On OTV-RTR2:

OTV-RTR2#sh otv overlay1
Overlay Interface Overlay1
 VPN name                 : None
 VPN ID                   : 1
 State                    : UP
 AED Capable              : Yes
 Join interface(s)        : GigabitEthernet0/0/1
 Join IPv4 address        : 10.70.0.2
 Tunnel interface(s)      : Tunnel1
 Encapsulation format     : GRE/IPv4
 Site Bridge-Domain       : 100
 Capability               : Unicast-only
 Is Adjacency Server      : No
 Adj Server Configured    : Yes
 Prim/Sec Adj Svr(s)      : 10.80.0.2

We can also see that the OTV adjacency is up:

OTV-RTR1#sh otv adj
Overlay 1 Adjacency Database
Hostname                       System-ID      Dest Addr       Up Time   State
OTV-RTR2                       c08c.6008.0f00 10.70.0.2       00:47:50  UP

We can also see that the URL on each router contains the opposite edge device:

OTV-RTR1#sh otv adjacency-server replication-list
Overlay 1 Unicast Replication List Database
Total num: 1

Dest Addr       Capability
10.70.0.2       Unicast

OTV-RTR2#sh otv adjacency-server replication-list
Overlay 1 Unicast Replication List Database
Total num: 1

Dest Addr       Capability
10.80.0.2       Unicast

AED

I mentioned a term in the previous post that I would like to revisit. The AED, or Authoritative Edge Device. To get some background, let’s reexamine our topology:

Sample OTV topology

In the first topology there is only one entry and exit point for overlay traffic. But in a production environment, you will likely have redundant edge devices. What happens then, when we add another OTV edge device to a site?

otv-topology-2

We’ll go ahead and add another router on the left side of the diagram, and call it OTV-RTR1A.  Now we have two interfaces on the same Vlan that don’t participate in STP, which means it’s possible for a loop to form. To prevent this from happening, edge devices at the same site will elect an AED, which is then the only device allowed to forward traffic for the overlay. This functionality applies to traffic going both directions — the AED will be the only device allowed to both encapsulate frames into IP packets towards the overlay, and decapsulate OTV packets and forward frames to the local LAN.

It’s also important to note that an AED will be elected per VLAN for automatic load-balancing. One edge device will be the AED for the even number VLANs, and the other for the odd VLANs. As far as I know, this is not configurable.

AED Election

Remember that site ID and site VLAN we configured in part 1? The Site ID is used to identify Edge devices in the same site, and the Site VLAN is used for communication between the edge devices. Another value, the OTV System ID, is used to elect the AED.

The System ID is a combination of the IS-IS system ID (viewed with the show otv isis protocol command) and the site identifier. You can view the OTV system ID with the show otv site command.

Using our new topology, let’s go back and add the site vlan to the inside interface on OTV-RTR1 and OTV-RTR1A (RTR1A has already been configred for OTV, using RTR1 as the adjacency server):

OTV-RTR1(config)#interface Gig0/0/0
OTV-RTR1(config-if)#service instance 100 ethernet
OTV-RTR1(config-if-srv)#encapsulation dot1q 100
OTV-RTR1(config-if-srv)#bridge-domain 100
OTV-RTR1(config-if-srv)#^Z
OTV-RTR1#

OTV-RTR1A(config)#interface Gig0/0/0
OTV-RTR1A(config-if)#service instance 100 ethernet
OTV-RTR1A(config-if-srv)#encapsulation dot1q 100
OTV-RTR1A(config-if-srv)#bridge-domain 100
OTV-RTR1A(config-if-srv)#^Z
OTV-RTR1A#

Now let’s verify that the two edge devices recognize each other, and that one has been elected AED.

OTV-RTR1#sh otv site
Site Adjacency Information (Site Bridge-Domain: 100)

Overlay1 Site-Local Adjacencies (Count: 1)

Hostname       System ID      Last Change Ordinal    AED Enabled Status
*OTV-RTR1      001E.4962.5400 00:02:20    0          site       overlay
 OTV-RTR1A     001E.F6B5.2600 00:02:20    1          site       overlay

OTV-RTR1A#sh otv site
Site Adjacency Information (Site Bridge-Domain: 100)

Overlay1 Site-Local Adjacencies (Count: 1)

Hostname       System ID      Last Change Ordinal    AED Enabled Status
OTV-RTR1       001E.4962.5400 00:02:39    0          site       overlay
*OTV-RTR1A     001E.F6B5.2600 00:02:39    1          site       overlay

The two devices have formed a ‘site-local adjacency’. Who’s the AED?

OTV-RTR1#sh otv vlan authoritative
Key:  SI - Service Instance

Overlay 1 VLAN Configuration Information
 Inst VLAN  Bridge-Domain  Auth  Site Interface(s)
 0    250   250            yes   Gi0/0/0:SI250
 Total VLAN(s): 1
 Total Authoritative VLAN(s): 1

OTV-RTR1A#sh otv vlan authoritative
Key:  SI - Service Instance

Overlay 1 VLAN Configuration Information
 Inst VLAN  Bridge-Domain  Auth  Site Interface(s)
 Total VLAN(s): 1
 Total Authoritative VLAN(s): 0

We can see that OTV-RTR1 is the AED at the site.

Failover

To test a failover scenario, we’ll simply shutdown the inside interface on OTV-RTR1. Once the interface is down, you’ll notice that OTV-RTR1A now considers itself authoritative for VLAN 250:

OTV-RTR1A#sh otv vlan authoritative
Key:  SI - Service Instance

Overlay 1 VLAN Configuration Information
 Inst VLAN  Bridge-Domain  Auth  Site Interface(s)
 0    250   250            yes   Gi0/0/0:SI250
 Total VLAN(s): 1
 Total Authoritative VLAN(s): 1

And when we examine the OTV event log, we see the following entry:

[09/16/13 19:44:24.724 23C9 490] OTV-APP-ISIS: AED set to UP for overlay 1 bd 250

Keep in mind that OTV is using IS-IS under the hood, so the failover processes are dependent on IS-IS timers.

Also, it’s important to note that the failover process does interrupt traffic. There is a short window of about 8-9 seconds where the remote OTV device doesn’t have an OTV route from the other site. It’s hard to grab the output exactly on time, though, so you’ll have to test in your environment to verify. In the outputs below, notice how the route for Sw-1 (0009.b709.4b80) is withdrawn and then is inserted again with OTV-RTR1A as the next hop:

OTV-RTR2#sh clock
14:05:05.745 MST Mon Sep 16 2013
OTV-RTR2#sh otv route

Codes: BD - Bridge-Domain, AD - Admin-Distance,
   SI - Service Instance, * - Backup Route

OTV Unicast MAC Routing Table for Overlay1

Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------
0    250 250   0009.b709.4b80 50    ISIS   OTV-RTR1
0    250 250   0009.b717.7880 40    BD Eng Gi0/0/1:SI250

OTV-RTR2#show clock
14:05:16.767 MST Mon Sep 16 2013
OTV-RTR2#sh otv route

Codes: BD - Bridge-Domain, AD - Admin-Distance,
    SI - Service Instance, * - Backup Route

OTV Unicast MAC Routing Table for Overlay1

Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------
0    250 250   0009.b717.7880 40    BD Eng Gi0/0/1:SI250

OTV-RTR2#show clock
14:05:25.800 MST Mon Sep 16 2013
OTV-RTR2#sh otv route

Codes: BD - Bridge-Domain, AD - Admin-Distance,
    SI - Service Instance, * - Backup Route

OTV Unicast MAC Routing Table for Overlay1

Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------
0    250 250   0009.b709.4b80 50    ISIS   OTV-RTR1A
0    250 250   0009.b717.7880 40    BD Eng Gi0/0/1:SI250

Conclusion

As you can see, OTV is still fairly simple to configure, but there’s a lot of stuff going on behind the scenes to make it work. Before you implement anything in production, be sure to have a good understanding of what’s actually happening.

Cisco OTV – Overlay Transport Virtualization

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First, let’s talk about what supports OTV — not much:

  • Nexus 7K
  • ASR 1K
  • CSR 1000V (For those of you not familiar with the Cloud Services router, I’d recommend reading this

What is OTV?

OTV is an encapsulation protocol that wraps L2 frames in IP packets in order to transport them between L2 domains. Typically this would be between remote datacenters, but it could also be within a datacenter if you needed an easy (expensive) way to extend a VLAN.

You will also see OTV referred to as ‘MAC Routing’, since the OTV devices are essentially performing routing decisions based on the destination MAC address in the L2 frame.

You might be thinking “Hey, I’ve already got this with EoMPLS and/or VPLS.” And you’d be right — you have the essence of what OTV accomplishes. What OTV adds, however, is simplicity and fault isolation.

When you configure OTV, you are defining 3 elements:

  • Join interface
    This is the interface that faces the IP core that will transport OTV encapsulated packets between sites.
  • Overlay InterfaceThis is the virtual interface that will handle the encapsulation and decapsulation of OTV packets sent between OTV edge devices.
  • Inside interface This is the interface that receives the traffic that will be sent across OTV.

What do I need before I can configure OTV?

Before you can setup OTV in your environment there are a few important details to know:

  • OTV adds 42 bytes of overhead into the packet header. This has implications if your MTU size is 1500 bytes (the default in most cases). You’ll need to either enable Jumbo frames across your core, or reduce the MTU size on your servers inside the OTV domain. UPDATE: You can enable OTV fragmentation by using the global command otv fragmentation join-interface.  I don’t know if this has any performance implications, but at least it’s an option for you if changing the MTU throughout your network is difficult.
  • With the latest code releases, I believe all platforms support either Unicast or Multicast for the OTV control-plane. If you have a multicast enabled core, use multicast — it’s really not too bad.

Topology and Configuration

For my topology I’m going to use two ASR 1K’s, a 4900M with two VRFs, and two 3550 switches. I know I could’ve left out the VRFs, but I wanted to make my topology as close as possible to real-life. So we end up with this:

Sample OTV topology

So let’s move on to the OTV configuration.

OTV Site information

Part of any OTV config will be defining the site identifiers and the Site Bridge-Domain. The site identifier is how an OTV device determines whether or not it is at the same location as another OTV device.

OTV-RTR1:

otv site-identifier 0001.0001.0001

OTV-RTR2:

otv site-identifier 0002.0002.0002

The site bridge-domain is the Vlan that the OTV edge devices at the same site will use for AED election. Since this VLAN will not be part of the overlay, we can use the same command on both routers.

otv site bridge-domain 100

The Join interface

The join interface will be the source for all OTV packets sent to remote OTV routers, and it will be the destination for OTV packets that need to come to the site. For multicast control-plane implementations you’ll need to enable Passive PIM and IGMPv3.

OTV-RTR1:

interface Gig0/0/1
mtu 8192
ip address 10.80.0.2 255.255.255.0
ip pim passive
ip igmp version 3

Also note that the MTU has been adjusted to accommodate the increased size of the OTV packet. This will be the same on the second OTV-RTR except for the IP address.

Overlay Interface

In the overlay interface configuration we have to specify the multicast group used for control messaging, as well as the range of multicast groups that will be used for passing multicast data within the VLAN. We will also specify which interface will be used as the join interface. This will be the same on both routers:

interface Overlay1
otv control-group 239.1.1.1
otv data-group 232.1.1.0/28
otv join-interface GigabitEthernet0/0/1
no shutdown

Once you turn up the Overlay interface on both sides, you should see your OTV adjacency form:

OTV-RTR1#show otv adjacency
Overlay 1 Adjacency Database
Hostname                       System-ID      Dest Addr       Up Time   State
OTV-RTR2                       c08c.6008.0f00 10.70.0.2       00:00:36  UP

At this point since there isn’t a Vlan bridged to the Overlay, there will be now OTV routing information:

OTV-RTR1#show otv route

Codes: BD - Bridge-Domain, AD - Admin-Distance,
       SI - Service Instance, * - Backup Route

OTV Unicast MAC Routing Table for Overlay1

 Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------

0 unicast routes displayed in Overlay1

----------------------------------------------------------
0 Total Unicast Routes Displayed

Adding Vlans to the Overlay

The last step will be to add the appropriate VLAN’s to the overlay. This config assumes that the router will receive the traffic from the switch with an 802.1Q tag:

interface GigabitEthernet0/0/0
service instance 250 ethernet
encapsulation dot1q 250
bridge-domain 250
!
interface Overlay1
service instance 250 ethernet
encapsulation dot1q 250
bridge-domain 250

Verifying

I created a Vlan interface on each switch to use as my ‘hosts’ for the ping tests.

Sw-1 VL250 = 0009.b709.4b80

Sw-2 VL250 = 0009.b717.7880

Pinging between devices is successful. Let’s look at the switches to see how it looks:

SW-1:

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 250    0009.b709.4b80    DYNAMIC     Gi0/1

OTV-RTR1:

OTV-RTR1#sh otv route

OTV Unicast MAC Routing Table for Overlay1

Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------
 0    250  250    0009.b709.4b80 50    ISIS   OTV-RTR2
 0    250  250    0009.b716.7880 40    BD Eng Gi0/0/0:SI250

So we can see that SW-1 knows to reach Sw-2 out interface Gi0/1, which connects to OTV-RTR1. OTV-RTR1 shows that it’s learned the MAC for SW-2 via OTV(ISIS) from OTV-RTR2. So anytime it receives frames for this MAC, it knows to forward them across the overlay.

OTV-RTR2:

OTV-RTR2#sh otv route

OTV Unicast MAC Routing Table for Overlay1

Inst VLAN BD     MAC Address    AD    Owner  Next Hops(s)
----------------------------------------------------------
0    250  250    0009.b709.4b80 40    BD Eng Gi0/0/0:SI250
0    250  250    0009.b716.7880 50    ISIS   OTV-RTR1

OTV-RTR2 shows that SW-2 is out the local service-instance. Any packets that come across the overlay will be decapsulated and forwarded out the local interface.

Wrap Up

Getting a basic OTV config up and running is not that difficult. Next time I’ll talk about using unicast instead of multicast, and also about AED.