Eduardo Collado - Blog

Committed Access Rate

BGP Attributes

A BGP attribute, or path attribute, is a metric used to describe the characteristics of a BGP path. Attributes are contained in update messages passed between BGP peers to advertise routes. There are four categories of BGP attributes. These are:

1) Well-known Mandatory
2) Well-known Discretionary
3) Optional Transitive
4) Optional Non-transitive

Optional transitive attributes may also be marked as partial.

(more…)

OSPF LSA Types

Type 1: Router link advertisements generated by each router for each area it belongs to. Flooded to a single area only.

Type 2: Network link advertisements generated by designated routers describing the set of routers attached to a particular network. Flooded to the area that contains th network.

Type 3/4: Summary link advertisements generated by ABRs describing inter-are routes. Type 3 describes routes to networks and is used for summarization. Type 4 describes routes to the ASBR.

(more…)

BGP Path Election

BGP uses the following criteria, in the order presented to select a path for a destination:

1. If the path specifies a next hop that is inaccessible, drop the update.
2. Prefer the path with the largest weight.
3. If the weights are the same, prefer the path with the largest local preference.
4. If the local preferences are the same, prefer the path that was originated by BGP running on this router.
5. If no route was originated, prefer the route that has the shortest AS_path.
6. If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than incomplete).
7. If the origin codes are the same, prefer the path with the lowest MED attribute.
8. If the paths have the same MED, prefer the external path over the internal path.
9. If the paths are still the same, prefer the path through the closest IGP neighbor.
10. Prefer the path with the lowest IP address, as specified by the BGP router ID.

Bridging Logic

A switch at the begining has no knowledge of any MAC address, so a switch don’t know what port to forward a frame, who this process works.

A switch learn in its CAM (Content Addressable Memory) MAC addresses reading the field Source Address in the Ethernet header, if the address is not in the CAM the switch learns this MAC address and stores in its CAM.

If the switch has no knowledge of where is that MAC address the switch forward the frame for all its ports until the switch receives response, once the swich has response, at that moment the switch stores its MAC address in its CAM and from that moment the switchs knows where is located that MAC address, so the switch only forward to all its ports a frame the once, the first time, after that our switch knows where to forward.

(more…)

Ethernet Type Fields

AS we saw at the post called “Ethernet Framing” there is a field called Type at Ethernet DIX and Length at the rest of ethernet types, and how Ethernet could distinguish it?

If this field is Length we know length of the Ethernet frame must be less than 1536 bytes, so that’s the way to determine if we are talkting about length or type, if the value in this field is greather than 1536 this field is a Type field if the value is less this field in a Length field, that is the easy way to distinguish it.

But fields DSAP and SSAP are used to distinguish the type, so if we have those fields really we have another field to write the type.

The values used in those fields are as following:

• Type: Those 2 bytes are administer by IEEE
• DSAP: This byte is registered and administer by IEEE
• SSAP: This byte is always the same value 0xAA if we are using 802.2, if we are using 802.1q the value must be 0×42 and so on, obviousy those values are registered and administer by IEEE

Ethernet Address’ Format

An Ethernet Address has 6 bytes long, the first 3 bytes are for Organisationally Unique Identifier (OUI) and the last 3 bytes are for the Network Interface Controller (NIC).

In ethernet addresses the most sifnificant byte is at left, but inside each byte most significant bit is at right side, the order of bits inside each byte is called canonical order or little-endian.

Also it is important to know I/G and U/L bits function.

• I/G bit, also known as the Individual/Group is used to know if the address is for unicast or multicast (0 = unicast, 1 = multicast), take notice this bit is the most significant bit in the most significant byte, so this bit it is located at right in the left byte in the ethernet address.
• U/L bit, AKA Universal/Local is used to know if the ethernet address is the assigned by the vendro or not (0 = assigned by the vendor, 1 = administratively assigned)

Often, when overriding the MAC address to use a local address, the device or device driver does not enforce the setting of the U/L bit to a value of 1.

Types of Ethernet / MAC Addresses

There are three types of ethernet address, Unicast, Broadcast and Multicast.

Unicast MAC Addresses has the I/G (Individual Group) bit set to 0
Multicast MAC Addresses has the I/G bit set to 1
Broadcast MAC Address is FF:FF:FF:FF:FF:FF

The I/G bit is the most significant bit in the First byte of the MAC address, that is all, quite easy.

Ethernet Framing

There are mainly 3 types or Ethernet frames, Ethernet DIX, IEEE 802.3 and IEEE 802.3 with SNAP header:

In the picture you can compare those three types of Ethernet Frames, right now we are going to talk about each field:

Preamble (8 or 7 bytes): This field is used for synchronization process and clock settings. This field is 7 bytes of 10101010 and one byte 10101011, but this last byte is only used in DIX version, in other versions this last byte it is replaced by a SD byte.
Start Delimiter - SD (1 byte): This byte means the end of the Preamble, this byte is always 10101011.
Destination Address (6 bytes): This field identifies the mac destination address of the frame.
Source Address (6 bytes): This field identifies the origin of the frame, the mac address of the origin of the frame.
Type / Portocol (DIX) (2 bytes): field that identifies the type of protocol or protocol header that follows the header. Allows the receiver of the frame to know how to process a received frame.
Length (802.3) (2 bytes): Describes the length, in bytes, of the data following the Length field, up to the Ethernet trailer. Allows an Ethernet receiver to predict the end of the received frame.
Destination Service Access Point (802.2) (1byte): DSAP; 1-byte protocol type field. The size limitations, along with other uses of the low-order bits, required the later addition of SNAP headers
Source Service Access Point (802.2) (1 byte): SSAP; 1-byte protocol type field that describes the upper-layer protocol that created the frame.
Control (802.2) (1 - 2 bytes): 1- or 2-byte field that provides mechanisms for both connectionless and connection-oriented operation. Generally used only for connectionless operation by modern protocols, with a 1-byte value of 0×03.
Organizationally Unique Identifier (SNAP) (3 bytes): OUI; 3-byte field, generally unused today, providing a place for the sender of the frame to code the OUI representing the manufacturer of the Ethernet NIC.
Type (SNAP) (2 bytes): 2-byte Type field, using same values as the DIX Type field, overcoming deficiencies with size and use of the DSAP field.

Configuring Cisco’s Switch Speed and Duplex

To configure a Cisco’s switch speed and duplex we only must to know a couple of commands:

speed {auto | 10 | 100 | 1000}
and
duplex {auto | half | full}

An example of configuration as follows

switch1(config)# int fa 0/13
switch1(config-if)# duplex full
switch1(config-if)# speed 100