Networking: IPv4/IPv6, IGPs, BGP, and route policy
What this domain tests
Networking covers IPv4 and IPv6 addressing, OSPF and IS-IS behavior, BGP path selection, and route policy. Exam items often combine a topology change with a policy: which adjacency forms, which prefix is filtered, or which path wins.
Core rules
- OSPF uses DR/BDR on broadcast networks and no DR on point-to-point links. Areas must touch area 0. Know LSA types 1, 2, 3, 4, 5, and 7.
- IS-IS uses L1 for intra-area, L2 for the backbone, and L1/L2 routers to leak routes. Wide metrics support larger values.
- BGP best-path order: weight, local preference, locally originated, AS path length, origin, MED, eBGP over iBGP, IGP metric to next hop, router ID. MED is compared only among paths from the same neighbor AS unless always-compare-med is configured.
- Route policy uses prefix lists, distribute lists, route maps, AS-path filters, and communities. Apply inbound policy to protect the router and outbound policy to influence neighbors.
- IPv6 relies on neighbor discovery, router advertisements, SLAAC, and DHCPv6. Dual-stack designs need separate policy and next-hop checks.
Worked example
Two eBGP paths reach 203.0.113.0/24. Path A has AS path 65001 65010 65020 (three ASes) and MED 200. Path B has AS path 65002 65030 65040 65050 (four ASes) and MED 50. Local preference is equal. BGP selects Path A because AS path length is compared before MED; the lower MED on B never matters. If the operator wants B for outbound traffic, set a higher local preference on B. That choice is local and is not advertised to eBGP peers. To influence inbound traffic from another AS, use AS-path prepending or MED, understanding that MED is a hint across different neighbor ASes.
Traps that cost marks
- Comparing MED across different neighbor ASes without always-compare-med.
- Forgetting that eBGP multihop needs a route to the next hop and often a next-hop-self change.
- Mixing OSPF virtual links and area design; a virtual link is not a general-purpose tunnel.
- Breaking IPv6 neighbor discovery with RA suppression or missing link-local addressing.