Why do many fabrics use eBGP for the underlay instead of OSPF or IS-IS?
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RFC 7938 describes it: one protocol for underlay and overlay, no flooding domain, a clean failure boundary on every link, per-neighbor policy, and AS-path loop prevention that stops bad paths in a Clos. It scales to very large fabrics and lets you drain a switch with a route-map. OSPF or IS-IS with iBGP EVPN on top is also common, especially in Cisco reference designs, and is simpler for small fabrics.
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Arguments for eBGP
- No flooding. A link flap in a link-state IGP floods LSAs everywhere and triggers SPF on every switch. In eBGP only the affected paths are updated.
- Policy. Route-maps and prefix lists per neighbor. You can drain a spine by prepending AS path.
- One protocol. The same BGP stack carries L2VPN EVPN for the overlay.
- Loop prevention by ASN. With all spines in one ASN, a route cannot go leaf, spine, leaf, spine.
What you must tune
- Detection. Default BGP timers are 60 s keepalive and 180 s hold. Use BFD, and rely on immediate session teardown when a directly connected link goes down.
- Multipath.
maximum-pathsmust be raised or there is no ECMP. - ASN plan. See the ASN scheme question. Bad plans cause path hunting or dropped routes.
When I would pick OSPF or IS-IS
A small or medium fabric, a team that knows OSPF well, or a vendor design that expects an IGP underlay with spines acting as iBGP route reflectors for EVPN. Both approaches work. Consistency matters more.
They will ask next
What does the underlay need to advertise?
Only loopbacks: the router ID loopback used for BGP EVPN peering and the VTEP loopback. Point-to-point subnets are optional, and BGP unnumbered removes them entirely.
Why is iBGP alone a poor underlay?
iBGP needs a full mesh or route reflectors, does not change the next hop by default and has no AS-path loop protection inside the AS. You end up needing an IGP underneath it anyway.
Traps
- Claiming eBGP converges faster by itself. Without BFD and fast link-down handling it is slower than a tuned IGP.