Citrix SD-WAN Routing, QoS, and Optimization
This domain tests how SD-WAN chooses a path and a queue for each flow. Questions cover routing protocols, virtual path selection, QoS classes, and WAN optimization, usually with a requirement such as protecting voice while bulk replication runs.
Routing
- SD-WAN supports static routes, OSPF, and BGP. Routes can point to virtual paths, link groups, or the underlay.
- Preferred routes and route metrics decide which path wins; per-packet and per-flow behavior matters for voice and for TCP performance.
- Overlay routes are learned or configured separately from underlay ISP routes; a missing route often looks like a path failure.
QoS
- Traffic is classified into QoS classes by application or policy, then shaped and scheduled per link. Bandwidth allocation can be hierarchical, with guaranteed and maximum rates.
- Priority queuing protects latency-sensitive traffic; shaping keeps a class from starving others.
- DSCP marking should be consistent end to end; mismatched marking causes the wrong queue on the next hop.
Optimization
- Packet duplication sends selected packets over multiple paths to survive loss; forward error correction adds parity without full duplication.
- WAN optimization uses compression, deduplication, caching, and TCP proxy. It helps bulk and repetitive traffic more than already-compressed or encrypted payloads.
- HDX optimization is policy-driven and should be scoped to ICA traffic, not all TCP.
Worked example
A 100 Mbps branch link carries voice, video, and backup. Allocate 10 Mbps guaranteed to voice with priority, 50 Mbps maximum to video with a lower priority, and 30 Mbps to backup with a maximum but no guarantee. Reasoning: guarantees protect voice during congestion; maximums stop backup from consuming the link; a priority queue alone without a maximum would let video starve backup completely.
Traps that cost marks
- Applying QoS on the wrong interface or direction, so egress policy never matches.
- Over-subscribing guaranteed bandwidth beyond the physical link rate.
- Assuming packet duplication is free; it consumes bandwidth on every selected path.
- Expecting deduplication to compress already encrypted or media streams.
- Forgetting route precedence between static and dynamic routes.