What happens when a router has two routing protocols running and is receiving similar routes from each routing protocol? Easy question, right? The routing protocol with the lower administrative distance has its routes installed in the routing table (routing information base – RIB). However, an interesting question that comes up from time to time is [...]
Outage Story with VTP
December 2, 2007 By 4 Comments
One of my accounts had an unfortunate network outage that lasted about an hour. This outage was caused by human error with VTP but not in the classic revision number way we have heard about before. Here is what happened… Related PostsTCP Incast and Cloud application performance Cisco UCS Networking videos (in HD), Updated & [...]
Switchport Configurations Explained
November 27, 2007 By 4 Comments
It always helps me to think of the English translation when trying to memorize and understand some the Cisco IOS settings I think are important. Here are some Cisco IOS switchport configurations translated into English: ‘switchport mode trunk‘ says: “Always trunk on this end, and I will send DTP to attempt to negotiate a trunk [...]
VLAN Trunking using IEEE 802.1Q
November 27, 2007 By 1 Comment
IEEE 802.1Q (sometimes referred to as 1Q or DOT1Q) is a industry standards based implementation of carring traffic for multiple VLANs on a single trunking interface between two Ethernet switches. 802.1Q is for Ethernet networks only. Unlike ISL , 802.1Q does not encapsulate the original Ethernet frame. For Ethernet V2 frames, 802.1Q inserts a new [...]
VLAN Trunking using ISL
November 26, 2007 By 3 Comments
Inter-Switch Link (ISL) is a Cisco specific implementation of trunking multiple VLANs between two Cisco switches where a single interface will carry traffic for more than one VLAN. ISL was designed to work with Ethernet, FDDI, Token Ring, and ATM. ISL completely encapsulates the original Ethernet frame by adding a new 26 byte header and [...]
Things to know about VTP
November 21, 2007 By 2 Comments
Some notes about VTP (VLAN Trunking Protocol): Cisco switches running Cisco IOS store VTP and VLAN information in a separate database stored in Flash, in file called vlan.dat. Cisco switches running CatOS store VTP and VLAN information in the main switch configuration file, stored in NVRAM. VTP information is only transmitted over trunk ports. A [...]
Identifying Ethernet Multicast
November 21, 2007 By 4 Comments
Just like there are 3 different Ethernet header types, there are also 3 different types of Ethernet addresses: Unicast Broadcast Multicast A unicast frame contains the unique MAC address of the destination receiver. A broadcast frame contains all binary 1′s as the destination address (FFFF.FFFF.FFFF). A multicast frame contains the unique multicast MAC address of [...]
Identifying Ethernet Header Types
November 19, 2007 By 4 Comments
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