FlipKart
Scenario (as per your description):
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Switch A is connected to:
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Switch B via 1 link
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Switch C via 1 link
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You are asking:
❓ Can I bundle these two links (to B and C) into a single port-channel?
Answer: ❌ No, it's NOT possible.
Reason:
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Port-channel (EtherChannel, LAG, LACP) requires:
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All member ports must terminate on the same remote device.
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Meaning: Both physical links in the port-channel must connect to the SAME switch on the other side.
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In your case:
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One link goes to Switch B.
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One link goes to Switch C.
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Since these are two different devices, you cannot form a standard port-channel.
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✅ Exception (in some special cases only):
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If Switch B and Switch C are configured in a special stacking or multi-chassis system like:
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Cisco vPC (Virtual Port Channel)
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StackWise (for stackable switches)
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VSS (Virtual Switching System)
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MLAG (in other vendors)
Then, from Switch A's view, B and C behave like a single logical switch, so you can create a port-channel to both B and C.
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But in normal standalone switches (not stacked/vPC): ❌ You cannot make a port-channel across two physical switches.
Summary:
| Situation | Port-Channel Possible? | Why? |
|---|---|---|
| Switch A → Switch B (multiple links) | ✅ Yes | Same destination switch |
| Switch A → Switch B & Switch C (single link each) | ❌ No | Different destination switches |
| Switch A → Switch B & C (but B & C are vPC/Stacked) | ✅ Yes | B & C act as a single logical switch |
🔍 DNS TTL (Time To Live) — Explained Simply:
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DNS TTL (Time To Live) is a setting in DNS records that tells resolvers (like your browser or ISP) how long to cache (store) the DNS response before asking the DNS server again.
✅ Key Points:
| Term | Meaning |
|---|---|
| TTL Value | Time (in seconds) the DNS record is valid in the cache. |
| Example | If TTL = 3600 seconds (1 hour), the resolver will cache that DNS result for 1 hour before checking again. |
🔍 What is Spine-Leaf Architecture?
A modern network topology designed for high scalability, low latency, and east-west traffic optimization—especially in Cisco Nexus and ACI environments.
✅ 1. Components:
| Layer | Description |
|---|---|
| Spine Switches | Core layer switches — connect to all leaf switches; never directly to other spines or endpoints. |
| Leaf Switches | Access layer switches — connect to servers, firewalls, storage, etc., and every spine switch. |
✅ 2. Key Characteristics:
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No Leaf-to-Leaf direct links.
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No Spine-to-Spine links.
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Each Leaf connects to every Spine — providing full any-to-any connectivity.
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Traffic between two Leaf switches always flows via Spine switches.
✅ 3. Why is this a "Class Architecture"?
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Separation of responsibilities:
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Leaf = Access/Distribution layer (Class 1)
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Spine = Core layer (Class 2)
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Highly structured design — "class" roles define functions strictly.
✅ 4. Advantages:
| Benefit | Why? |
|---|---|
| Scalable | Add more leafs without changing spines. |
| Predictable Latency | All paths are the same number of hops (leaf → spine → leaf). |
| Redundant | Multiple paths ensure high availability. |
| East-West Traffic Friendly | Ideal for server-to-server (VM to VM) communication inside Data Centers. |
✅ 5. Typical Cisco Devices Used:
| Layer | Cisco Examples |
|---|---|
| Spine | Nexus 9000 Series (Core) |
| Leaf | Nexus 9000/7000/5000 (Access/Distribution) |
✅ Diagram:
✅ One-Line Interview Definition:
"Spine-Leaf is a class-based modern Data Center architecture where all Leaf switches connect to all Spine switches to provide scalable, high-performance east-west traffic with predictable low-latency paths."
📌 Your Scenario:
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2 Spines: S1, S2
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5 Leafs: L1, L2, L3, L4, L5
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L4 has loopback 1.1.1.1/32 configured (or maybe any endpoint behind it).
Your question is:
👉 How does L1 know that 1.1.1.1/32 is reachable via L4?
🔍 What happens in a typical Spine-Leaf Fabric?
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Routing protocol is running (usually eBGP/iBGP, OSPF, or IS-IS) between:
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Each Leaf ↔ Spine link.
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Spines don’t connect to each other.
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Leafs don’t connect directly to each other.
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✅ Step-by-Step Route Learning:
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L4 advertises 1.1.1.1/32 to both Spines (S1 & S2) via BGP (or other protocol).
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S1 & S2 receive the route, and now know "1.1.1.1/32 is reachable via L4".
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S1 & S2 then advertise this route to all Leafs (L1, L2, L3, L5).
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L1 receives the route from S1 and S2, learns that:
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"1.1.1.1/32 is reachable via S1 and S2" — next-hops are the Spines, not L4 directly.
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But BGP carries additional information, like the BGP AS-Path or next-hop IP, that indicates the original source is L4.
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🔍 How does L1 "know" it came from L4?
✅ If using eBGP EVPN or iBGP:
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BGP carries the next-hop attribute — usually L4's Loopback or interface IP.
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So the next-hop value points to L4's IP — though the actual packet must first reach the spine, then leaf4.
✅ If using OSPF/IS-IS:
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The route carries the originating router ID, which belongs to L4.
✅ In Cisco ACI or VXLAN-Fabric:
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Uses BGP EVPN control-plane:
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L4 advertises 1.1.1.1/32 via EVPN Route-Type 5.
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The originator ID (L4) is included in the BGP EVPN route.
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Leafs (L1) know who originated the route based on this information.
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✅ Important Concept:
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Data-plane (Actual Traffic): L1 will send the packet towards S1/S2 (spines), because traffic has to flow via spines — unless ACI/VXLAN allows direct leaf-leaf routing (called "direct routing").
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Control-plane (Routing Info): L1 knows the origin (L4) from BGP attributes like next-hop or originator-ID.
📝 Final Interview Answer:
"In a spine-leaf fabric using BGP or OSPF, L4 advertises the route 1.1.1.1/32 to both spines. The spines propagate this route to all other leaf switches, like L1. In the routing update, L1 receives the next-hop or origin information pointing to L4, so L1 knows that 1.1.1.1/32 is originated from L4, but traffic must reach L4 via either spine switch as no direct leaf-to-leaf links exist."
🔍 What is TCP MSS?
MSS (Maximum Segment Size) is a value in the TCP header that defines:
"The largest segment of data (in bytes) that a device is willing to receive in a single TCP segment."
✅ MSS excludes the TCP and IP headers.
✅ It only counts the payload (data) part.
✅ Formula for MSS:
🔍 Why is MSS important?
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Prevents fragmentation:
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MSS ensures the TCP segments stay within the MTU limit to avoid fragmentation at Layer 3.
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Used during TCP 3-way handshake:
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Both hosts exchange their MSS values using TCP Options field.
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Each side agrees to use the smaller MSS value for the connection.
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In network devices (like routers/firewalls):
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We sometimes set or adjust MSS (called MSS Clamping) to prevent PMTUD (Path MTU Discovery) issues, especially over VPN or tunnels.
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✅ Example:
✅ Command Example (Cisco):
🔍 Common Interview Summary Answer:
"TCP MSS defines the largest data segment (excluding TCP/IP headers) that a device can accept. It's used during TCP handshake to avoid fragmentation and is typically 1460 bytes on Ethernet networks. In networks with tunnels or VPNs, MSS is often adjusted (clamped) to ensure smooth packet flow without fragmentation."
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