Shortest Path Bridging (SPB) and Fabric Operations
SPB operation questions test your ability to read the fabric's control plane: IS-IS adjacencies, nicknames, B-VIDs and learned I-SIDs.
Control plane
- SPB uses IS-IS to build a link-state database; VSPs form adjacencies only on NNI ports where IS-IS and SPBM are enabled.
- Each node advertises a system ID and a nickname. The nickname must be unique; a duplicate makes forwarding ambiguous and shows up as unreachable services.
- I-SID advertisements tell every BEB which remote B-MAC terminates a service, so a local VLAN-to-I-SID mapping resolves to a backbone destination.
Data plane
- Unicast follows the shortest path using IS-IS metrics; equal-cost paths are used for ECMP.
- Multicast and broadcast inside an I-SID use a tree rooted at the ingress BEB, so the fabric core needs no STP.
- Encapsulation adds overhead, so NNI MTU must be at least as large on every hop; mismatched MTU produces 'small pings work, applications fail' symptoms.
Verification sequence
show isis adjacencies (are neighbours up?), show isis spbm (nickname, B-VID, system ID), show i-sid (local and remote I-SID bindings), show isis interface (which ports are actually in the fabric).
Worked example
Three VSPs form a triangle: A-B, B-C and A-C. VLAN 10 on A maps to I-SID 10100, and VLAN 110 on C maps to the same I-SID. On A, show isis adjacencies shows two Up adjacencies, show isis spbm shows a unique nickname and B-VID 4051 on all nodes, and show i-sid 10100 shows the local VLAN 10 binding plus a remote B-MAC belonging to C. Hosts ping. Had C kept a different B-VID, the adjacency might still form but I-SID 10100 would not resolve - which is why you check B-VID and MTU before blaming the access port.
Traps
Enabling IS-IS on UNI ports; mismatched B-VID or MTU; duplicate nicknames; assuming a link that is physically up has an adjacency up; expecting STP to block one leg of the triangle (SPB uses both); and forgetting that a BCB must run SPBM but must not hold the service.