network-architect

name	network-architect
description	Network architecture and engineering — routing (BGP, OSPF, EIGRP), switching (STP, VLANs, 802.1Q, EtherChannel), WAN (MPLS, SD-WAN, DMVPN, IPsec), QoS, redundancy (HSRP/VRRP/GLBP), IPv4/IPv6 addressing and subnetting, and network troubleshooting. Use when designing topology, reviewing routing policy, troubleshooting L2/L3, or producing network architecture documentation.

Network Architect Skill

Covers network design, routing/switching, WAN, and troubleshooting at CCNP depth.

Ryan's context: CCNA (2003–2006), CCNP R&S (BSCI/BCMSN/ISCW/ONT), Applied Computer Science. Currently designing enterprise SASE solutions.

Shared skills referenced:

• Zero Trust / SASE → ../zero-trust-architect/SKILL.md
• Network security controls → ../network-security/SKILL.md
• DNS tools → ../dns-tools/SKILL.md

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IP Addressing & Subnetting

VLSM Quick Reference

Prefix	Hosts	Typical use
/30	2	Point-to-point links
/29	6	Small DMZ / loopbacks
/28	14	Small office
/27	30	Medium VLAN
/26	62	Large VLAN
/25	126	Large campus segment
/24	254	Standard site VLAN
/22	1022	Supernet / summary

RFC 1918 Private Address Space

10.0.0.0/8        — Class A (large enterprise, data centres)
172.16.0.0/12     — Class B (medium enterprise)
192.168.0.0/16    — Class C (branch, home office)

IPv6 Address Types

Type	Prefix	Use
Global Unicast	2000::/3	Routable internet
Link-Local	fe80::/10	On-link only; auto-assigned
Unique Local	fc00::/7	Private (analogous to RFC 1918)
Multicast	ff00::/8	Replaces broadcast
Loopback	::1/128	Local loopback

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Routing Protocols

OSPF

Key design rules:
- Area 0 (backbone) required; all other areas connect to it
- Use stub/totally stub for areas with single ABR
- Summarise at ABR boundaries (area range) and ASBR (summary-address)
- BFD for sub-second convergence on high-value links
- OSPF authentication: MD5 minimum; SHA-256 preferred (OSPFv3)

Troubleshooting checklist:
1. Are MTU values matching on both sides? (ip mtu mismatch → EXSTART/EXCHANGE stuck)
2. Are hello/dead timers matching?
3. Is the subnet mask identical?
4. Are area IDs matching?
5. Stub area flag mismatch?

BGP

Key design rules:
- eBGP: always set next-hop-self on iBGP reflectors
- Use prefix-lists (not distribute-lists) for filtering — faster, cleaner
- Null-route aggregates before advertising summaries (ip route x.x.x.x 255... null0)
- AS-path prepending for inbound traffic engineering (not overused)
- Set MED for outbound preference signal to upstream
- BGP communities for policy tagging across eBGP peers

Path selection order (remember: N WLLA OMDI):
  Next hop reachable → Weight (Cisco-local) → Local-Pref → Locally originated
  → AS-path length → Origin (IGP < EGP < ?) → MED → eBGP > iBGP
  → IGP metric to next-hop → Router-ID (lowest)

Security hardening:
- TTL Security (GTSM): neighbor x ttl-security hops 1
- MD5 authentication on all eBGP sessions
- max-prefix with threshold + warning

EIGRP

Feasibility condition: FD of successor > RD of feasible successor
Stuck-In-Active (SIA): query propagation too wide → use stub or summarisation
Named mode EIGRP preferred for configuration clarity

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Switching

STP Variants

Variant	Standard	Notes
STP	802.1D	Legacy; slow (30–50s convergence)
RSTP	802.1w	Fast convergence (~1s); backward-compatible
PVST+	Cisco	Per-VLAN STP; load balancing via root placement
Rapid PVST+	Cisco	RSTP per VLAN; current default on Cisco
MST	802.1s	Maps multiple VLANs to instances; scale

STP hardening:
- PortFast on all access ports
- BPDU Guard on PortFast ports
- Root Guard on uplinks where root must not appear
- Loop Guard on non-designated ports (replaces UniDirectional Link Detection)
- BPDU Filter: use only on ports where STP BPDUs are explicitly unwanted

VLANs and Trunking

802.1Q trunk: native VLAN must match on both sides (or use vlan dot1q tag native)
DTP: always disable on access ports (switchport nonegotiate)
VTP: use transparent mode or VTPv3 with password; avoid VTP v1/v2 in large environments

VLAN design conventions:
  VLAN 1   — never use for data; isolate or shut unused ports
  VLAN 99  — management VLAN (or any non-1 native)
  VLAN 10  — data
  VLAN 20  — voice (with QoS trust dscp on IP phones)
  VLAN 30  — management devices
  VLAN 100+ — service VLANs

EtherChannel

LACP (802.3ad): preferred; interoperable
PAgP: Cisco-only
Static (on/on): use only when LACP unavailable

Load balancing hash options (balance on most variable field):
  src-mac / dst-mac / src-dst-mac
  src-ip  / dst-ip  / src-dst-ip   ← recommended for routed traffic
  src-port / dst-port / src-dst-port ← recommended for server farms

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WAN Technologies

MPLS L3VPN

PE–CE routing: eBGP, OSPF, or static
Route distinguisher (RD): makes VPN routes unique in MP-BGP (not security)
Route target (RT): controls VPN import/export (the actual policy)
VRF: per-customer routing table on PE

Troubleshooting:
  show bgp vpnv4 unicast all summary
  show ip route vrf <name>
  show mpls ldp neighbor
  show mpls forwarding-table

IPsec VPN

IKEv2 preferred (vs IKEv1 — less round-trips, better re-keying)
Encryption: AES-256-GCM
Integrity: SHA-256 or SHA-384
DH group: 14 minimum; 19/20 (ECDH) preferred
PFS: always enable (new DH exchange per SA)
Dead Peer Detection (DPD): always enable

Transform sets to avoid: DES, 3DES, MD5, DH group 1/2/5

DMVPN

Phase 1: hub-to-spoke only (spoke cannot talk directly to spoke)
Phase 2: spoke-to-spoke (NHRP redirect; one NHRP resolution per pair required)
Phase 3: spoke-to-spoke with NHRP shortcut (summarise at hub; more scalable)

SD-WAN (Cisco Viptela / Catalyst SD-WAN)

Control plane: vManage (GUI/API), vSmart (policy), vBond (orchestration)
Data plane: vEdge / cEdge with OMP (Overlay Management Protocol)
Policy types: centralized (control/data) vs local (access/device)
Application-aware routing: based on BFD SLA metrics (jitter, loss, latency)

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QoS

DSCP Marking Reference

Class	DSCP Decimal	Binary	Use
EF	46	101110	VoIP RTP (voice bearer)
CS3	24	011000	Call signalling
AF41	34	100010	Video conferencing
AF31	26	011010	Mission-critical data
AF11	10	001010	Bulk data
CS1	8	001000	Scavenger (low priority)
BE	0	000000	Best effort (default)

Trust boundary: always at the IP phone or first managed switch port
Mark at edge; never remark in the core
VoIP sizing rule: BW = (Codec_bps + 160b_overhead) × calls
CBWFQ + LLQ: queue voice in LLQ (priority); everything else in CBWFQ classes

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Gateway Redundancy

Protocol	Standard	VIP ownership	Load balance
HSRP v2	Cisco	Active router	No (one active)
VRRP	RFC 5798	Master router	No
GLBP	Cisco	AVG + multiple AVF	Yes (per-host round-robin)

Best practice: HSRP or VRRP for simplicity; GLBP when upstream bandwidth balance matters
Object tracking: track interface/IP SLA; decrement priority on failure to trigger failover

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Troubleshooting Methodology (OSI Bottom-Up)

L1  Physical   — cabling, SFP, interface status (show int, show cdp neigh)
L2  Data Link  — MAC table, ARP, STP state, trunk config (show mac address-table, show spanning-tree)
L3  Network    — routing table, ARP, ping, traceroute (show ip route, debug ip routing)
L4  Transport  — port reachability (telnet <ip> <port>), access-list hits
L7  App        — DNS resolution, TLS handshake, application logs

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NETWORK_KNOWLEDGE

Append discoveries here. Format: YYYY-MM-DD: <note>