Advertising Routes into BGP · Lesson 01 of 01
Advertising routes into BGP
There are four ways to originate a prefix, and they are not interchangeable.
Written lesson
The network command
network does not create a route — it advertises one that already exists in the routing table, exactly, mask and all. network 192.0.2.0 mask 255.255.255.0 advertises 192.0.2.0/24 only if a /24 for that prefix is already installed by some other means (connected, static, or an IGP). If the exact match is missing, BGP originates nothing, which is the single most common "why isn't my prefix advertised" cause.
Configuration
network, with the matching route present
ip route 192.0.2.0 255.255.255.0 Null0
!
router bgp 65001
address-family ipv4 unicast
network 192.0.2.0 mask 255.255.255.0Written lesson
Redistribution and its risk
redistribute pulls routes from an IGP or connected/static into BGP wholesale. It is powerful and dangerous: without a route-map filter it can inject thousands of internal prefixes, and it sets ORIGIN to incomplete. Prefer network for the handful of prefixes you mean to announce, and reserve redistribution for cases where the set of prefixes is large and dynamic — always with a filter.
Four ways, ranked
- `network` — precise, exact-match required, ORIGIN igp. Preferred for announced prefixes.
- `redistribute` — bulk, risky, ORIGIN incomplete. Filter it.
- `aggregate-address` — summarise several prefixes into one (its own chapter).
- Default origination — `neighbor default-originate` sends a default without it being in the table.