2 · Path attributes¶

Every BGP route carries attributes. Policy sets them, the best-path algorithm compares them. Understanding them is most of understanding BGP.

Here they are on a real route from the lab fabric:

BGP routing table entry for 172.16.30.0/24
 Paths: 1 available
  65002
    10.0.13.3 from 10.0.13.3 (3.3.3.3)
      Origin IGP, metric 0, localpref 100, IGP metric 0, weight 0, tag 0
      Received 00:29:38 ago, valid, external, best

65002 is the AS path. Then origin, MED (shown as metric), local preference, weight — and the verdict: valid, external, best.

The four categories¶

The categories aren't trivia — they define what a router must do with an attribute it doesn't recognise, which is what let BGP gain features like communities without breaking older routers.

Category	Must support?	Pass on if unknown?
Well-known mandatory	yes — must be in every update	n/a
Well-known discretionary	yes	n/a
Optional transitive	no	yes, marked partial
Optional non-transitive	no	no, dropped

Attribute	Category
ORIGIN, AS_PATH, NEXT_HOP	well-known mandatory
LOCAL_PREF, ATOMIC_AGGREGATE	well-known discretionary
COMMUNITY, extended communities	optional transitive
MED, ORIGINATOR_ID, CLUSTER_LIST	optional non-transitive

Why optional transitive matters

A router that doesn't understand communities still forwards them, so a community you set survives crossing networks that don't use it. That property is what makes communities usable as an inter-provider signalling mechanism.

MED is non-transitive by design — it's only meaningful between two directly adjacent ASes, so it shouldn't leak further.

AS_PATH¶

The list of ASes a route has crossed. Two jobs:

Loop prevention. A router rejects any route whose AS path already contains its own AS. This is why eBGP doesn't need a separate loop mechanism — and why iBGP, which doesn't prepend, needs the full-mesh rule instead.

Path length, step 4 of best-path. Shorter usually wins.

Prepending is the standard way to make a path less attractive:

route-map PREPEND permit 10
 set as-path prepend 65001 65001 65001

Advertise the same prefix to two providers, prepending on the backup, and inbound traffic prefers the shorter path.

Prepending is a blunt instrument

You're asking the entire internet to prefer another path, and anyone with local preference set on your prefixes ignores you completely — local-pref is compared at step 2, prepending at step 4.

Prepending influences; it does not control. Communities that a provider has agreed to honour are more precise.

NEXT_HOP¶

Where to send traffic for this prefix.

eBGP — the advertising router sets it to itself
iBGP — passed along unchanged

That second rule is Lab 01's trap. An iBGP peer receives a next hop belonging to a link in someone else's AS, has no route to it, and the route is unusable.

The route must be usable, not just present. BGP considers a route valid if the next hop resolves at all — including via a default route, which in a lab is often the management interface. See Lab 01 step 4.

Fix with next-hop-self on the border router.

LOCAL_PREF¶

Which exit this AS should use. Higher wins. Default 100.

Sent between iBGP peers, never to eBGP peers — it's an internal decision, so it propagates through your AS and stops at the boundary.

route-map PREFER_PRIMARY permit 10
 set local-preference 200

It's compared at step 2, so it beats AS path, MED and everything below. That makes it the tool for "always use provider A while it's up," regardless of what the paths look like.

Local-pref controls outbound traffic — your own routers use it to choose an exit. It cannot influence what other people send you.

MED¶

Which entrance to use, when there are several into your AS. Lower wins.

The mirror of local-pref: local-pref is your decision about leaving, MED is a suggestion to your neighbour about entering.

Two things make MED awkward:

It's only compared between paths from the same neighbouring AS by default. Comparing MEDs from different providers is meaningless — they're not the same scale — though always-compare-med will do it if you insist.
It's a request, not an instruction. Your neighbour's local-pref is compared first and overrides it entirely.

If a provider ignores your MED, that's normal; MED is step 6.

WEIGHT¶

Local to one router, never advertised. Highest wins, default 0 for learned routes.

Originally Cisco, now widely implemented — Arista shows it in the output above.

Because it's step 1 and purely local, weight is a blunt override for a single router. Prefer local-pref for anything that should apply AS-wide: weight affects one box and silently does nothing on the others, which makes for confusing troubleshooting.

ORIGIN¶

How the route entered BGP. Compared at step 5, preference order:

Code	Means
i (IGP)	a `network` statement — most preferred
e (EGP)	historical, effectively extinct
? (incomplete)	redistributed — least preferred

Seen as Origin IGP in the capture above, and as a trailing i in show ip bgp.

The practical consequence: a redistributed route (?) loses to an otherwise equal route from a network statement. If two paths tie until step 5 and the "wrong" one wins, check whether one side is redistributing.

COMMUNITY¶

A tag you attach to a route. It does nothing by itself — its value is that policy elsewhere can match on it.

Written AS:value, e.g. 65001:100.

Well-known values:

Community	Effect
`NO_EXPORT`	don't advertise outside the AS (or confederation)
`NO_ADVERTISE`	don't advertise to any peer
`LOCAL_AS`	don't advertise outside the local sub-AS

route-map TAG_CUSTOMER permit 10
 set community 65001:100

The real power is provider signalling: a transit provider publishes communities like "set local-pref 80 on my routers" or "don't advertise to AS X," and you tag your announcements accordingly. That gives precise control where prepending only gives a nudge.

Which attribute for which job¶

You want to control	Use	Direction
Which exit we use	LOCAL_PREF	outbound traffic
Which entrance they use	MED, prepending, communities	inbound traffic
One router's local choice	WEIGHT	that router only
Stop a route spreading	COMMUNITY (`NO_EXPORT`)	advertisement scope

The asymmetry that trips people up

You control what you send out easily, and what comes in only by persuasion.

Local-pref is authoritative — your routers obey it. Anything influencing inbound traffic (MED, prepending, communities) is a request to networks you don't operate, and they may have policy that overrides it.

Next: Best-path selection → — the order all this is compared in.