Routing Switching

---
name: routing-switching
description: "Cross-platform comparison and architecture guidance for all routing and switching technologies: L2/L3 design, BGP, OSPF, STP, VLAN architecture, VXLAN/EVPN, and campus/DC fabric design. Use for \"routing protocol\", \"switching\", \"BGP vs OSPF\", \"VLAN design\", \"STP\", \"spine-leaf\", \"campus network\", \"EVPN-VXLAN\", \"ECMP\", \"VRF\". Do NOT use for cisco-ios-xe-, cisco-nxos-, arista-eos-, juniper-junos-, aruba-aoscx-, or meraki-specific configuration -- use that technology's own skill."
license: MIT
---

# Routing & Switching

This skill covers all routing and switching technologies, providing cross-platform expertise spanning Cisco IOS-XE, Cisco NX-OS, Arista EOS, and general routing/switching fundamentals. Sibling technology skills provide deep implementation details.

## When to Use This Skill vs. a Technology Skill

**Use this skill when the question is cross-platform, comparative, or architectural:**
- "Compare OSPF vs BGP for my data center underlay"
- "Should I use spine-leaf or three-tier?"
- "How does STP root bridge election work?"
- "Design a VLAN strategy for 2000 users"
- "Explain EVPN-VXLAN symmetric IRB"
- "When to use VRF vs firewall zones for segmentation?"

**Route to a technology skill when the question is platform-specific:**
- "Configure BGP EVPN on a Nexus 9000" --> the `cisco-nxos` skill
- "Catalyst 9300 StackWise configuration" --> the `cisco-ios-xe` skill
- "Arista MLAG with VXLAN setup" --> the `arista-eos` skill
- "IOS-XE 17.12 upgrade procedure" --> the `cisco-ios-xe` skill's `references/versions/17.12.md`
- "NX-OS vPC fabric peering" --> the `cisco-nxos` skill

## How to Approach Tasks

1. **Classify** the request:
   - **Architecture / Design** -- Load `references/concepts.md` for protocol fundamentals
   - **Platform comparison** -- Compare relevant platforms, then route for implementation
   - **Troubleshooting** -- Identify the layer (L2 vs L3), then consult the appropriate technology skill
   - **Configuration** -- Consult the specific technology skill
   - **Migration** -- Assess source/target platforms, load concepts for protocol compatibility

2. **Gather context** -- Topology type (campus, DC, WAN), scale (endpoints, VLANs, routes), existing equipment, traffic patterns (north-south vs east-west), redundancy requirements

3. **Analyze** -- Apply routing/switching principles. Consider convergence, scalability, operational complexity, and failure domains.

4. **Recommend** -- Provide specific, actionable guidance with platform trade-offs

5. **Qualify** -- State assumptions about topology, scale, and traffic patterns

## Platform Comparison

### When to Use Each Platform

| Platform | Best For | Key Strengths |
|---|---|---|
| **Cisco IOS-XE** | Campus networks, branch/WAN, SD-Access | Broadest campus portfolio, Catalyst Center, Wi-Fi integration, StackWise |
| **Cisco NX-OS** | Data center spine-leaf, VXLAN/EVPN fabrics | vPC, VXLAN/EVPN maturity, Nexus Dashboard, ACI option |
| **Arista EOS** | Data center fabrics, cloud-scale, hyperscaler | Sysdb resilience, CloudVision, AVD automation, eAPI programmability |

### CLI Differences

| Task | IOS-XE | NX-OS | Arista EOS |
|---|---|---|---|
| Enable feature | Always available | `feature bgp` required | Always available |
| Save config | `write memory` | `copy run start` | `write memory` |
| Checkpoint/rollback | `configure replace` | `checkpoint` / `rollback` | `configure checkpoint` |
| API | RESTCONF/NETCONF | NX-API | eAPI (JSON-RPC) |
| Telemetry | gNMI/MDT | gRPC/gNMI | gNMI + Sysdb |
| Management platform | Catalyst Center | Nexus Dashboard (NDFC) | CloudVision (CVP/CVaaS) |

### Routing Protocol Support

All three platforms support BGP, OSPF, IS-IS, EIGRP (Cisco only), static routing, PBR, and VRF. Key differences:

- **EIGRP**: Cisco-only (IOS-XE and NX-OS). Not available on Arista EOS.
- **EVPN-VXLAN**: All three support it. NX-OS and EOS are most mature for DC fabrics. IOS-XE added campus EVPN in 17.12+.
- **Segment Routing**: IOS-XE and NX-OS support SR-MPLS and SRv6. EOS supports SR-MPLS.
- **BFD**: All three support hardware-offloaded BFD for sub-second failover.

## Architecture Selection

### Campus Networks

| Architecture | Scale | Platforms | Notes |
|---|---|---|---|
| Collapsed core | <500 endpoints | IOS-XE (Cat 9300/9500) | Two switches, L3 at core, simple STP domain |
| Three-tier | 500-10K endpoints | IOS-XE (Cat 9200/9300/9400/9500) | Core + distribution + access, STP bounded at distribution |
| SD-Access fabric | Any scale | IOS-XE + Catalyst Center | LISP/VXLAN overlay, SGT policy, automated provisioning |
| Arista campus | Any scale | EOS (720XP, 756) | BGP-based campus, CloudVision managed |

### Data Center Networks

| Architecture | Scale | Platforms | Notes |
|---|---|---|---|
| Spine-leaf (2-tier) | <48 leaf pairs | NX-OS (Nexus 9000) or EOS (7050X/7060X) | VXLAN/EVPN, eBGP underlay |
| Super-spine (3-tier) | >48 leaf pairs | NX-OS or EOS | Add super-spine tier for scale |
| Multi-site DCI | Multiple DC fabrics | NX-OS BGW or EOS Border Gateway | EVPN multi-site with Type-5 re-origination |
| ACI | Policy-driven DC | NX-OS (ACI mode) + APIC | Intent-based, EPG/Contract model |

### Underlay Protocol Selection (DC)

| Protocol | Best For | Trade-offs |
|---|---|---|
| eBGP | Modern spine-leaf (recommended) | Policy control, AS-path loop prevention, scales well |
| OSPF | Smaller fabrics, simpler ops | LSDB size concern at scale, no built-in policy |
| IS-IS | Large-scale fabrics | Fast convergence, no LSDB flooding storms, preferred NX-OS underlay |

## Redundancy Technologies Comparison

| Technology | Platform | Max Members | Use Case |
|---|---|---|---|
| StackWise | IOS-XE (Cat 9200/9300) | 8 | Campus access layer physical stacking |
| StackWise Virtual | IOS-XE (Cat 9400/9500/9600) | 2 | Campus distribution/core logical stacking |
| vPC | NX-OS (Nexus 9000) | 2 | DC leaf pair active-active to servers |
| MLAG | Arista EOS | 2 | DC leaf pair active-active to servers |
| EVPN MH (ESI-LAG) | NX-OS / EOS | 4+ | Standards-based multi-homing, no peer-link |
| HSRP/VRRP | IOS-XE / NX-OS / EOS | 2 (active/standby) | First-hop gateway redundancy |

## Technology Routing

| Request Pattern | Route To |
|---|---|
| Cisco IOS-XE, Catalyst switches, ISR/ASR routers, SD-Access, Catalyst Center | the `cisco-ios-xe` skill |
| IOS-XE 17.12 LTS specifics | the `cisco-ios-xe` skill's `references/versions/17.12.md` |
| IOS-XE 17.18 current release specifics | the `cisco-ios-xe` skill's `references/versions/17.18.md` |
| Cisco NX-OS, Nexus switches, vPC, DC VXLAN/EVPN, Nexus Dashboard | the `cisco-nxos` skill |
| NX-OS 10.5 maintenance release specifics | the `cisco-nxos` skill's `references/versions/10.5.md` |
| NX-OS 10.6 feature release specifics | the `cisco-nxos` skill's `references/versions/10.6.md` |
| Arista EOS, eAPI, CloudVision, AVD, MLAG | the `arista-eos` skill |
| EOS 4.35 current train specifics | the `arista-eos` skill's `references/versions/4.35.md` |

## Common Pitfalls

1. **Mixing L2 and L3 boundaries** -- Keep L2 domains bounded. Extend L2 across sites only via VXLAN, never by stretching VLANs over trunks across a WAN.
2. **STP as architecture** -- STP is a safety net. Design to minimize STP dependence. In DC fabrics, eliminate STP entirely with routed spine-leaf.
3. **Ignoring MTU for VXLAN** -- VXLAN adds 50 bytes overhead. Set fabric links to MTU 9214 or VXLAN encapsulated frames will be fragmented or dropped.
4. **Mismatched EVPN route targets** -- RT import/export must match across all VTEPs for a given VNI. A single mismatch silently breaks connectivity.
5. **No BFD on fabric links** -- Without BFD, BGP/OSPF failover depends on hold timers (seconds to minutes). BFD detects failures in milliseconds.
6. **EIGRP in multi-vendor** -- EIGRP is Cisco-proprietary. Never use it in a multi-vendor environment. Use OSPF or BGP instead.

## Reference Files

- `references/concepts.md` -- BGP path selection, OSPF areas/LSA types, STP variants, VLAN design, ECMP, VRF, EVPN-VXLAN fundamentals. Read for cross-platform architecture and protocol questions.