What are the STP times?

 Hello Timer  Forward delay Timer  Max-age Timer   Hello Timer: How often switches send BPDU’s by default every 2 seconds Forward delay Timer: how much long a port must spend time in both learning and listening state. By default 15 seconds Max-age Timer: How long a switch will retain BPDU information from a neighbor switch [...]

Why we use Spanning Tree Protocol in the Switches?

 Broadcast Storms Duplicate Frame copies Unstable MAC Table   Broadcast StormsWithout any loop removing mechanism, switches will flood broadcasts endlessly throughout the network. This is known as broadcast storm. Duplicate Frame copiesA device could receive duplicate copies of same frame from different switches. It creates additional overhead on the network. Unstable MAC TableWhen switch receives [...]

What are the link aggregation protocols in Cisco Switches, what are their modes required to bundle a link?

Dynamic Configuration Cisco switches support two dynamic aggregation protocols: PAgP (Port Aggregation Protocol) – Cisco proprietary aggregating protocol. LACP (Link Aggregation Control Protocol) – IEEE standardized aggregation protocol, originally defined in 802.3ad. PAgP and LACP are not compatible – both sides of an Etherchannel must use the same aggregation protocol. EthernChannel – PAgP It supports [...]

In Cisco Switches for VTP, what frame tagging protocol they support?

Cisco switches support two frame tagging protocols: Inter-Switch Link (ISL) IEEE 802.1Q Inter-Switch Link (ISL) It is Cisco’s proprietary frame tagging protocol.  It encapsulates a frame with an additional header (26 bytes) and trailer (4bytes). It increases the size of a frame by 30 bytes. The header contains several fields, including a 15-bit VLAN ID. [...]

Are OSPF routing protocol exchanges authenticated?

Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication may be through simple passwords or through MD5 cryptographic checksums. To configure simple password authentication for an area, use the command ip ospf authentication-key to assign a password of up to eight octets to each interface attached to the area. Then, issue the area x [...]

Useful BGP commands on Cisco Routers

When BGP is not behaving correctly, a “trick” to temporarily stop peering with a neighbor is to use the following command: router bgp 194 neighbor<ipaddress> password xxx Since the other router doesn’t have the same password, the two routers will stop talking to one another, without you having to do anything else. Later, when the [...]

What is Control Plane & Data plane?

The control plane in general is anything that’s needed in order to get routing working on that device; in other words, it is the “signalling” of the network. Control plane packets are destined to or locally originated by the router itself. This is really what separates the concept of the control and data plane. For [...]

BGP Quick notes

A transit AS is an AS that routes traffic from one external AS to another external AS   The “show ip bgp” command is used to display entries in the BGP routing table.   The AS-PATH attribute is used to prevent BGP routing loops. When receiving an BGP advertisement, the router checks the AS-PATH attribute, [...]

Passive interface (RIP,OSPF,EIGRP)

Passive-interface command is used in all routing protocols to disable sending updates out from a specific interface. However the command behavior varies from o­ne protocol to another. RIP: In RIP this command will disable sending multicast updates via a specific interface but will allow listening to incoming updates from other RIP enabled neighbors.This simply means [...]

Quick notes for EIGRP

EIGRP  is  a Cisco proprietary protocol which means it will work only on Cisco routers. EIGRP is also called advanced distance vector or Hybrid routing protocol. Multicast or unicast is used for exchange of information. Multiple network layer protocols are supported. 100% loop-free. By default,EIGRP will limit itself to use no more than 50% of [...]

Memorising the BGP decision making process

1. Weight (Bigger is better) 2. Local preference (Bigger is better) 3. Self originated (Locally injected is better than iBGP/eBGP learned) 4. AS-Path (Smaller is better) 5. Origin 6. MED (Smaller is better) 7. External (Prefer eBGP over iBGP) 8. IGP cost (Smaller is better) 9. EBGP Peering (Older is better) 10. Router- ID

What is Multi VLAN port ?

The multi-VLAN port is a feature which allows switch for configuring a single port for two or more vlans. This feature allows users from different VLANs to access a server or router without implementing InterVLAN routing capability. A multi-VLAN port performs normal switching functions in all its assigned VLANs. Multi-VLAN port will not work when [...]

Is it EIGRP support secondary ip address

Yes the Enhanced Interior Gateway Routing Protocol (EIGRP) supports secondary addresses. However, because EIGRP supply data packets from the primary address, all the routers should be configured with the primary addresses that belong on the same subnet. Be sure that the primary address on the interface is configured for EIGRP by issuing the network command [...]

What is BGP backdoor?

In BGP it is a component that is used to modify the admin distance of eBGP in order for an interior portal routing protocol (IGP) to take precedence over an eBGP route. By default,External BGP (eBGP) has an admin distance value of 20. Administrative distance is the first criterion that a router uses to determine [...]

What is error disable state? How to recover from error disable port?

The ErrDisable feature is implemented to handle critical situations where the switch detected excessive or late collisions on a port, port duplex misconfiguration, Ether Channel misconfiguration, Bridge Protocol Data Unit (BPDU) port-guard violation, UniDirectional Link Detection (UDLD), and other causes. The error-disable function let the switch to shut down a port when it encounters physical, [...]

How to use the embedded wireshark in Cisco 3850 switches to capture the traffic

On the 3850 switches has embedded wireshark that can be used to packet capture during the troubleshooting this negate the need of SPAN to capture the traffic. -Define your source monitor capture mycap interface GigabitEthernet1/0/1 both -Set your match statement monitor capture mycap access-list myacl monitor capture mycap match ipv4 any any -Set your destination [...]

Is it possible to use distribute-list command to filter routes in OSPF?

Configuring distribute-list commands that utilize route maps will not achieve desired Link State Advertisement (LSA) filtering. Open Shortest Path First Protocol (OSPF) routes cannot be filtered from entering the OSPF database using distribute-list. The distribute-list in command only filters routes from entering the routing table, and it does not prevent LSA packets from being propagated. [...]