OSPF gives you four different ways to stop a prefix from getting somewhere, and they are not interchangeable. Two of them remove the LSA from the link-state database. One leaves the LSA in place and only touches the local
Here is a routing puzzle. An OSPF router has a route to an external network. The route is in the routing table. The next hop is a device that is not the router that advertised it and is not even
You need to reload a core router. It is carrying live traffic. The naive approach - just reload it - drops every packet in flight and triggers a reconvergence storm. The slightly-less-naive approach - shut its interfaces one at a time
Look at your routing table and count how many of those prefixes anyone actually sends traffic to. In a typical service provider or large enterprise core, a large fraction of the OSPF routing table is transit link subnets - the /30s
OSPF converges fast. That is its whole reason for existing. But "fast" is a tuning decision, and the knobs are not where most people look. They are not the hello and dead timers - those control failure detection. The
The CCIE troubleshooting section does not ask you to explain BGP. It hands you a broken network and a clock. The difference between passing and failing is not knowing more BGP - it is having a repeatable method for turning a
Here is a problem that a great many service providers had, and some still have: a fully working, fully tuned, IPv4-only MPLS core, and customers who want IPv6. Upgrading every P router in the core to dual-stack is
BGP installs one best path. That is the default and it is deliberate: BGP is a policy protocol, and the best-path algorithm exists to produce exactly one winner. But if you have two circuits to the same provider, one
Most networks use BGP communities the way most people use a junk drawer: things get put in, nobody agrees what they mean, and after two years nobody dares throw anything away. That is a shame, because a well-designed community
Dampening penalises a prefix for flapping and refuses to use it even after it recovers. Captured live on CML with IOS XE 17.18.2: penalty 2818 from three flaps, the prefix held at *d while an identical prefix from the same neighbor stays best, plus the advertisement-interval gotcha that makes...
Inbound prefix filtering is normal hygiene: you accept the routes you want and drop the rest. But think about what actually happens on the wire. Your neighbour sends you a full table, your router receives every prefix, parses every attribute,
Every dual-homed network eventually asks the same question: how do I keep a backup path truly idle until I need it? Local preference and AS-path prepending only change which path is preferred. The backup prefix is still out