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Category: ARP

Directed ARP and ICMP Redirects

One of my readers sent me this question:

When I did my ***redacted*** I encountered a question about Directed ARP. The RFC ( is in the "experimental" stage, and I found it really weird from ***** to include such a hidden gem in the ***redacted***.

Directed ARP is clearly one of those weird things that people were trying out in the early days of networking when packet forwarding and bandwidth were still expensive (read the RFC for more details), but I kept wondering “what exactly is going on when a host receives an ICMP redirect?” Time for a hands-on test.

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ARP Processing in Layer-3-Only Networks

John Jackson wrote an interesting comment on my Rearchitecting L3-Only Networks blog post:

What the host has configured for its default gateway doesn't really matter, correct? Because the default gateway in traditional L2 access networks really isn't about the gateway's IP address, but the gateway's MAC address. The destination IP address in the packet header is always the end destination IP address, never the default gateway.

He totally got the idea, however there are a few minor details to consider.

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Optimal Layer-3 Forwarding with Active/Active VRRP (Enterasys Fabric Routing)

Enterasys implemented optimal layer-3 forwarding with an interesting trick: they support VRRP like any other switch vendor, but allow you to make all members of a VRRP group active forwarders regardless of their status.

Apart from a slightly more synchronized behavior, their implementation doesn’t differ much from Arista’s Virtual ARP, and thus shares the same design and deployment caveats.

For more information, watch the Fabric Routing video from the Enterasys Robust Data Center Interconnect Solutions webinar.

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Arista EOS Virtual ARP (VARP) Behind the Scenes

In the "Optimal L3 Forwarding with VARP and Active/Active VRRP" blog post I made a remark along the lines of "Things might get nasty [in Arista EOS Virtual ARP world] if you have configuration mismatches", resulting in a lengthy and amazingly insightful email exchange with Lincoln Dale during which we ventured deeper and deeper down the Virtual ARP (VARP) rabbit hole. Here's what I learned during out trip:

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Follow-up: Interface default route

Judging by your comments, some of you have already faced a stupidity similar to the one I’ve described on Friday (BTW, I’ve remembered this particular debacle when receiving a Pingsta case invitation with very similar symptoms). The symptoms are well described in the comments: the CPU utilization of the ARP process increases, packet forwarding becomes sluggish and the router runs out of memory, potentially resulting in a router crash. Now let’s analyze what’s going on.

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My stupid moments: Interface default route

Years ago I was faced with an interesting challenge: an Internet customer was connected to our PE router with an Ethernet link and I did not want to include the PE router’s IP address in the default route on the CE router.

The latest IOS release in those days was probably somewhere around 11.x; none of the DHCP goodies were available.

After pondering the problem for a while, I got a brilliant idea: if I would use an interface default route, proxy-ARP would solve all my problems. This is the configuration I’ve deployed on the CE-router:

interface Ethernet 0
description Uplink to the ISP
ip address
ip route ethernet 0

We tested this configuration in the middle of the night and it worked as expected. What do you think happened in the morning?

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Generating layer-2 broadcast from a regular IP packet

The Wake-on-LAN discussion we had a while ago brought us nowhere; there's simply no way to generate UDP packets on the router. I thought I could use Application Performance Monitor's Tcl scripts to generate the packet, but it looks like APM has been removed from recent IOS releases (and it's not clear whether you can use APM without a peer router).

The discussion nonetheless had an interesting side effect. Robert Turnšek sent me an interesting trick: with static ARP you can generate layer-2 broadcasts with a layer-3 unicast packet.

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ARP timeout resolution is implemented in minutes

Under some circumstances, you might want to tune the ARP timers on the router (for example, when using ARP as a keepalive mechanism to detect whether the host is up). Unfortunately, although you can set the per-interface arp timeout in seconds, the actual timer resolution is in minutes. For example, if you set the ARP timeout to 10 seconds, the router will age the ARP entries once per minute.
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ARP entries are periodically refreshed if you use CEF switching

In a previous post I've been writing about the inability to clean the ARP cache due to cached CEF adjacencies. As it turns out, this behavior has another side effect: the router will automatically refresh all ARP entries (and CEF adjacencies) as they expire from the ARP cache. This might become a problem on high-end devices with a lot of directly connected hosts if you set the arp timeout to a low value.

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