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title: ARP and Static Routes « ipSpace.net blog | daily.dev
description: A hands-on exploration of how routers use ARP to resolve MAC addresses for different types of static route configurations. Using a virtual lab with two Linux...
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og:description: A hands-on exploration of how routers use ARP to resolve MAC addresses for different types of static route configurations. Using a virtual lab with two Linux...
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# ARP and Static Routes « ipSpace.net blog

**[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 7 min read · 0 upvotes · 0 comments

## Summary

A hands-on exploration of how routers use ARP to resolve MAC addresses for different types of static route configurations. Using a virtual lab with two Linux hosts and an Arista vEOS switch, the post covers three static route scenarios: next-hop-only routes, interface-only routes (on unnumbered links), and combined interface+next-hop routes. It demonstrates how Arista EOS handles recursive next-hop resolution, what happens when a next hop is not in the routing table (route marked with '%', not installed in kernel), and how Cisco IOS-XE rejects ARP replies for such routes as 'wrong cable'. A bonus 'Baron Munchausen' trick shows that a static route's next hop can be within the same prefix it defines, and still work correctly on EOS.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://blog.ipspace.net/2023/08/arp-static-routes>

## Questions this post answers

### What source IP does Arista EOS use in ARP requests when a static route points to an unnumbered interface?

When a static route points to an unnumbered interface on Arista EOS, the router sends an ARP request using the IP address of the loopback interface that is loaning its address to the outgoing interface. For example, with a route like 'ip route 10.1.0.5/32 ethernet 2', the ARP request shows 'who-has 10.1.0.5 tell 10.0.0.1', where 10.0.0.1 is the loopback address assigned to the unnumbered interface.

_Network engineers configuring unnumbered interfaces track ARP quirks like this on daily.dev._

### What happens on Arista EOS when a static route has a next hop that is not in the routing table?

Arista EOS accepts the static route configuration but does not install it in the Linux kernel routing table, marking it with a '%' symbol in 'show ip route' output along with a warning that some routes are not programmed in the kernel. EOS still attempts ARP resolution for the next hop, but because the route is absent from the kernel table, locally generated traffic (e.g., ping) does not work, though ASIC-forwarded packets might still be forwarded.

_Engineers troubleshooting static route installation issues on Arista EOS find edge cases like this on daily.dev._

### Can a static route's next hop be within the same IP prefix the route defines on Arista EOS?

Yes, on Arista EOS a static route's next hop can fall within the same prefix it defines — for example, 'ip route 10.1.0.5/32 ethernet 2 10.1.0.5'. This is technically equivalent to specifying only the outgoing interface without a next hop, so EOS installs it as a directly connected route in both the EOS routing table and the underlying Linux kernel, and connectivity works normally.

_Networking engineers exploring Arista EOS routing behavior find practical lab results like this on daily.dev._

## Similar posts on daily.dev

- [The Never-Ending IPv6 Loopback Prefix Saga « ipSpace.net blog](https://daily.dev/posts/the-never-ending-ipv6-loopback-prefix-saga-ipspace-net-blog-4nlhgh10l) · ipSpace.net Blog · 0 upvotes · 0 comments

---

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