Datacom: IGP, BGP, MPLS/VXLAN and IPv6 Behaviour on VRP
What this domain tests
Datacom questions check whether you can design, verify and troubleshoot a mid-size enterprise network built on Huawei VRP devices. Content clusters into four blocks: IGP behaviour (OSPF and IS-IS), BGP path control, MPLS L3VPN with VXLAN/EVPN as the modern overlay, and IPv6 plus basic automation. Expect single-statement judgement items and short topology scenarios where one command output decides the answer.
Core rules to hold
- OSPF: ABRs generate one type 3 LSA per source area. Type 7 exists only inside an NSSA and is translated to type 5 at the ABR when the P-bit is set. Totally stubby areas filter types 3 (except a default), 4 and 5.
- IS-IS: Level-1 routers reach other areas only via a Level-1-2 router's ATT bit; DIS election has no BDR, and wide metrics are required for TE.
- BGP best path order: weight → local preference → locally originated → shortest AS_PATH → lowest origin → MED → eBGP over iBGP → lowest IGP metric → oldest route → lowest router ID.
- MPLS forwarding: push at the ingress PE, swap in the core, pop at the penultimate hop (PHP). VPNv4 routes carry an RD for uniqueness and RTs for import/export.
- VXLAN: 24-bit VNI; VTEPs encapsulate; EVPN type 2 carries MAC/IP, type 3 carries the IMET (inclusive multicast) route.
Worked example
Two iBGP paths to 10.1.1.0/24 are received. Path A: local preference 200, AS_PATH length 3, origin incomplete, MED 10. Path B: local preference 100, AS_PATH length 2, origin IGP, MED 5. The router selects Path A, because weight (equal) and local preference are compared first, and 200 beats 100 - AS_PATH length, origin and MED are never reached.
Traps that cost marks
- Comparing MED between paths learned from different neighbouring ASes; by default MED is only compared within the same neighbour AS.
- Assuming Local_Pref propagates across eBGP sessions - it is exchanged only between iBGP peers.
- Forgetting that a BGP next hop must be resolvable by an IGP before the route becomes valid.
- Mixing up the 12-bit VLAN ID with the 24-bit VNI.