Do explosion-proof telephones support static routing?

A phone can register during commissioning, then fail in the field because the plant network is segmented and the route is wrong. That creates missed calls and repeat site visits.

Yes, many explosion-proof SIP telephones support static routing, but the depth varies. Basic static routes and one default gateway are common. Full policy routing for SIP/RTP/provisioning is usually handled by the network, not the endpoint.

Static routing on Ex SIP phones: what is realistic?

What most Ex phones can do

In industrial deployments, “static routing support” usually means one of these capabilities:

• One default gateway on the active interface

• A small list of static IPv4 routes (destination + mask + gateway)

• VLAN tagging (802.1Q ¹) for a dedicated voice VLAN

• Dual DNS servers, and sometimes domain search rules

This is enough for many plants. If the phone sits in a voice VLAN that can reach the SIP registrar ² and NTP server, the phone works. Static routes become useful when the voice VLAN is restricted and only a few destinations are allowed.

Where endpoint routing often stops

Many endpoints do not behave like routers. Common limits show up:

• Only one default gateway is supported at a time

• No real policy-based routing (PBR) by application or port

• Limited route table size (sometimes single digits)

• No per-service binding (example: “SIP uses gateway A, provisioning uses gateway B”)

This is not a flaw. It is a design choice. Phones are designed to be predictable endpoints, not L3 devices.

A simple decision framework

Static routing on the phone makes sense when it reduces complexity. It does not make sense when it tries to replace the plant core network design.

Need in the project	Static routes on the phone help?	Better place to solve it
Reach SIP server in another subnet	Yes	Phone static route or L3 switch route
Reach provisioning server only	Yes	Phone static route + ACL
Separate “voice” and “control” paths	Sometimes	L3 core with VRF or PBR
SIP over one uplink, RTP over another	Rare	SBC/media relay + network policy
Dual gateways and failover logic	Limited	Redundant gateway (VRRP 3/HSRP) + DNS/SIP failover

A small story from a refinery project fits here. One unit had two subnets on the same switch. The phone registered fine, but firmware updates never worked. The issue was a missing static route to the provisioning server. Adding one route fixed it, and the plant stopped doing manual updates.

Static routing is powerful when it is used to reach a few controlled services. It becomes risky when it tries to outsmart the network.

Keep reading. The next sections translate this into clear tender wording and a deployment method that scales.

Can per-interface routes be set with dual LAN, VLANs, and separate voice/control subnets?

A dual-LAN phone looks like an easy way to split traffic. In practice, it can become confusing if routes and gateways are not defined clearly.

Some explosion-proof telephones can set routes per interface when they truly support dual LAN as separate interfaces. Many models still treat one port as a bridge or pass-through. The safest approach is one “voice” interface with VLAN tags, and a controlled management path handled by the network.

Understand what “dual LAN” means on the specific model

Dual LAN can mean three different things:

1) Two independent NICs: each has its own IP settings. This is best for separate subnets.

2) Switch + PC pass-through: the phone has one IP stack, and the second port is just a bridge.

3) Bonding/failover: rare on rugged SIP endpoints, but it exists in some industrial designs.

Only the first case supports true per-interface routing. The second case is not routing. It is switching.

VLANs are the cleanest segmentation tool for most plants

If the switch supports VLANs, it is usually simpler to:

• Put the phone on a voice VLAN (tagged)

• Put management tools on a management subnet that can reach the phone through defined ACL rules

• Avoid mixing “control” and “voice” on the endpoint itself

This keeps the phone config small and keeps the routing logic in the plant L3 design.

When per-interface routing can still be useful

Per-interface routes help when:

• The voice network is isolated, but the phone must reach one management server in another subnet

• The phone has a second dedicated management port that goes to a different switch segment

• The site wants out-of-band maintenance during shutdown windows

In that case, keep routing simple. Use static routes for only the required destinations, and avoid two default gateways.

Design pattern	What it looks like	Why it works	What to avoid
Voice VLAN only	One interface, tagged VLAN	simplest, scalable	exposing the voice VLAN to everything
Voice + limited mgmt route	One interface + 1–3 static routes	controlled access	big route tables on endpoints
True dual NIC (rare)	LAN1 voice, LAN2 management	clear separation	two default gateways or “smart” PBR
Pass-through port	Phone + PC on same drop	saves cabling	assuming the pass-through is a second routed interface

A good tender line for dual LAN is:

• “Dual LAN shall be either true independent interfaces with separate IP settings, or clearly documented as pass-through. If independent, device shall support static routes per interface.”

This wording prevents the most common misunderstanding.

How are route metrics, default gateways, and policy routes managed for SIP, RTP, and provisioning?

Plants often want “SIP uses one path, provisioning uses another.” That is a policy routing problem, and endpoints often do not implement it fully.

Most Ex SIP phones use standard longest-prefix match with one default gateway. Route metrics and policy routing are limited on endpoints. The practical method is to route by destination: add static routes for SIP registrar, NTP, and provisioning servers, and let the network handle any complex PBR needs.

Default gateway behavior: keep it predictable

Most phones behave like this:

• If the destination is in the local subnet, send direct

• If not, use the best matching static route

• If no match, use the default gateway

So, if the phone must reach three services outside the subnet, you can add three static routes. This keeps traffic controlled and avoids accidental internet reachability.

Metrics: do not rely on them unless the phone proves it

Some devices allow a metric value. Many do not. Even when they do, the metric may only be used in tie-break cases. The safest approach is:

• Use clear destination prefixes

• Avoid overlapping routes on the phone

• Keep one default gateway

SIP vs RTP: why policy routing is tricky on endpoints

SIP signaling is usually easy. It goes to the registrar or proxy. RTP is harder because media can go:

• direct endpoint-to-endpoint (peer-to-peer RTP), or

• through an SBC ⁴ or PBX media relay

If the plant needs strict path control, a media relay design is cleaner. It forces RTP to a known set of IPs. Then static routes can be destination-based and stable.

Provisioning and DNS: keep discovery aligned with routes

Provisioning over HTTPS ⁵ often uses an FQDN. That means DNS reachability matters. If the phone can reach the provisioning server by route, but cannot reach DNS, it still fails.

A clean design is:

• Route to SIP server(s)

• Route to DNS server(s)

• Route to NTP server(s)

• Route to provisioning server(s)

Service	What to route to	Why it matters	Simple endpoint rule
SIP registrar/proxy	fixed IPs or FQDN + DNS	registration stability	static routes to SIP + DNS
RTP media	SBC/media relay IP range	avoids one-way audio	force media to relay if needed
Provisioning	HTTPS server + DNS	fast replacement	static route to provisioning + DNS
NTP	local NTP server 6	log accuracy	static route to NTP if isolated

A practical tender line that fits most plants:

• “Device shall support static routes for SIP registrar/proxy, provisioning server, DNS, and NTP. Complex policy routing shall be handled by the network.”

This avoids a long debate about per-port PBR on endpoints.

Is static routing deployed via web UI, DHCP options, or API templates for large rollouts?

Manual routing setup does not scale. It also creates drift, because each technician leaves a slightly different configuration.

Static routes can be set in a web UI for pilots, but large rollouts should use auto-provisioning templates or a central management method. Some networks use DHCP to deliver routes, but endpoint support varies, so provisioning is the most reliable fleet approach.

Web UI: fine for proof, risky for fleets

Web UI is useful for:

• testing one phone on one VLAN

• validating which route fields exist

• confirming behavior after reboot

It is not good for:

• hundreds of phones

• multi-site change control

• audit trails

Provisioning templates: the best industrial approach

Most industrial SIP deployments already use provisioning for:

• SIP accounts and registrar addresses

• VLAN and QoS settings

• certificates for HTTPS/TLS

• feature behavior (hotline, paging, keys)

Static routes fit well into the same template. It also allows:

• per-site profile files

• version control

• rollback if a route breaks access

DHCP-delivered routes: useful when supported, but not universal

There are DHCP options that can provide classless routes. In theory, this is clean. In reality, many SIP endpoints either do not support it or implement it inconsistently across firmware builds. In plants, inconsistent behavior is worse than manual work.

So the safer plan is:

• Use DHCP for IP address and Option 42 for NTP if you want

• Use provisioning for routes and SIP behavior

• Use SNMP/syslog for monitoring

Method	Best scale	Strength	Main risk
Web UI	1–10 units	fast tests	drift and human error
Provisioning	10–10,000 units	consistent and auditable	needs template discipline
DHCP routes	10–10,000 units	central network control	endpoint support varies
API / config push	100–10,000 units	automation friendly	security and change logging needed

A practical rollout sequence that works:

1) Configure one pilot phone by web UI

2) Export or mirror settings into the provisioning template

3) Factory reset a second phone and confirm it builds itself correctly

4) Roll out in batches by site or VLAN

This sequence catches routing mistakes before they hit the whole fleet.

What security and reliability gains do static routes provide with 802.1X, ACLs, and redundant SIP servers?

A plant network is not only about connectivity. It is about controlling what the device can reach, and proving it stays stable under faults.

Static routes can reduce attack surface and improve uptime by limiting reachability to only required services. When combined with 802.1X, VLAN segmentation, and switch/firewall ACLs, they help enforce a “phone can only talk to PBX, DNS, NTP, and provisioning” model and support clean failover to redundant SIP servers.

Security gains: tight reachability by design

Static routes help security in a simple way. They prevent “default gateway to everything” behavior when you do not want it. That supports goals like:

• No internet access from the voice VLAN

• No lateral movement to OT subnets

• Only approved services are reachable

Still, routes alone are not security. They must be paired with ACLs. The best pattern is:

• Static routes define allowed destinations

• ACLs enforce allowed ports and directions

• 802.1X controls which device can join the VLAN

Reliability gains: fewer surprises during network changes

Dynamic routing in the core is fine. The endpoint should stay simple. Static routes keep endpoint behavior stable during:

• firewall policy updates

• new VLAN segmentation projects

• temporary routing changes during shutdown work

For redundancy, focus on these designs:

• Use redundant SIP servers with DNS SRV or configured primary/secondary

• Use a redundant default gateway on the VLAN (VRRP/HSRP at the network level)

• Keep routes minimal so failover paths are predictable

How static routes fit with 802.1X

802.1X ⁷ can place a phone into the correct VLAN and apply the right role. Static routes then ensure the phone reaches only the allowed services inside that role. This reduces the chance that a mis-tagged port exposes the phone to a larger network.

Control objective	What static routes add	What else is still required
Limit reachability	only route to required servers	ACLs and firewall rules
Reduce lateral risk	no route to peer subnets	VLAN segmentation and port security
Support secure provisioning	route only to HTTPS server	TLS, cert policy, and DNS controls
Improve failover	stable path to redundant SIP	redundant gateways and SIP failover config
Audit readiness	predictable traffic paths	logging, syslog, and change control

A practical security tender line that buyers like:

• “Endpoint shall support static routes to limit reachability, and shall be deployable in a voice VLAN with 802.1X and ACL enforcement.”

This makes it clear that static routes are part of a layered defense, not the only defense.

Conclusion

Yes, Ex SIP phones can support static routes, mainly for destination-based control. For a routing-ready OEM template and BOM, reach Jason Mark at info@sipintercommanufacturer.com.

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Footnotes