“PSTN” means “Public Switched Telephone Network,” and “ISDN” means “Integrated Services Digital Network.”
1.PSTN lines are analogue while ISDN lines are digital.
2.When comparing the two networks, the PSTN lines are used for small companies and ISDL are used for bigger companies.
3.The ISDN provides 128 kbps, which is really good for the Internet. PSTN (maximum rate of 56kbps) has a disadvantage that it does not make the most possible use of the broadband.
4.While PSTN does not allow two simultaneous connections, it is allowed in ISDN service.
5.When using ISDN, one can make faster calls than when using the PSTN.6.Both are mostly obsolete now with Ethernet and TCP/IP based networks and the Internet providing all that and a lot more.
A network administrator has a situation that requires time-based ACLs. Users are not allowed to access the Internet during business hours, except during lunch and after hours until 7 p.m. when the office closes. This is a time-based ACL that supports the requirement:
R1(config)# time-range employee-time
R1(config-time-range)# periodic weekdays 12:00 to 13:00
R1(config-time-range)# periodic weekdays 17:00 to 19:00
R1(config)# access-list 100 permit ip 192.168.1.0 0.0.0.255 any time-range employee-time
R1(config)# access-list 100 deny ip any any
R1(config)# interface FastEthernet 0/1
R1(config-if)# ip access-group 100 in
In this example, the commands allow IP access to the Internet during lunch time and after work hours. ACL 100 permits employee traffic to the Internet during lunch and after work hours between 5 PM and 7 PM.
NOTE – on point-to-point links a DR and BDR are not elected since only two routers are directly connected.
On LANs, DR and BDR have to be elected. Two rules are used to elect a DR and BDR:
1. Router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1.
2. If there is a tie, a router with the highest router ID wins the election.
3. If there is a tie again, a router with the highest loopback interface address wins the election.
4. If there is still a tile, a router with the highest active physical interface address wins the election.
***The router with the second highest OSPF priority or router ID will become a BDR.
Follow these guidelines and restrictions when configuring EtherChannel interfaces:
Spanning Tree Protocol (STP) is essential protocol in redundant switching topologies. The main purpose of this protocol is preventing switching loops. Frames does not have TTL field, so it’s not possible to discard frames, which loops continuously. There are many implementation of STP (PVST, PVST+, rapid PVST+, RSTP, MSTP), but fundamentals are the same for all of them.
Every redundant switching topology is expose to broadcast storms risk. Every broadcast frame will loop into infinity crating broadcast of unwelcome and very high traffic.
To check, how broadcast storms influence your switch CPU usage disable STP for one of VLANs and send one broadcast frame on this VLAN (ex. arp request):
no spanning-tree vlan 1
The first step in STP is choosing the RootBridge. It’s the switch with the lowest BridgeID (build from STP priority and switch MAC address) in whole network.
To change BridgeID and have influence on RoodBridge election, you can change STP priority of switch (by default 32768).
Example of changing STP priority for VLAN 1:
(config)# spanning-tree vlan 1 priority ?
Each STP process steps are calculated for each VLAN, so you can choose different priority for different VLANs and have different RootBridge in each VLAN.
Each non RootBridge switch have to choose one port, which will be named RootPort in STP terminology. If more VLANs exists on switch, RootPort will be chosen for every VLAN separately (except VLANs, where this switch is RootBridge).
RootPort is the port, which will lead frames through the fastest path to the RootBridge. To calculate the best path STP use summary costs to RootBridge for each path.
Cost is the function of Ethernet connection speed:
If there are two (or more) ports which have the same best path cost to the RootBridge, switch will choose port, which lead to RootBridge through the lowest BridgeID neighbor switch. If the neighbor switches have the same BridgeID (consider two switches connected with two Ethernet cables on different ports) lower STP port priority decide, which port will be seleted as RootPort. If the priority is the same for both ports, the lowest port number will be chosen.
To change interface cost (should be changed on both ends of cable):
(config-if)# spanning-tree cost ?
To change interface STP port priority:
(config-if)# spanning-tree port-priority ?
Every RootPort forward all frames and do not block any traffic.
From Ethernet segment perspective, STP have to choose one Designated Port for each segment. This port should lead frames to the RootBridge with the lowest cost (the fastest, best path).
STP choose blocked port role for every port that has not been named yet. This ports do not forward common Ethernet traffic.
STP process take some time and is longer on bigger switched networks. Every port is going through several states (blocking, listening, learning, forwarding) until forwarding frames and this process can take even 50 seconds. This can cause some problems for PCs connected to switch, because only after this period switch gives PC NIC possibility to talk with other devices.
That’s why PortFast, enhancement Cisco technology, gives the possibility to immediately changing PC ports to forward state in STP networks.
(config-if)# spanning-tree portfast
To ensure, that PortFast port will not cause Layer2 loops, enable bpduguard on this port also:
(config-if)# spanning-tree bpduguard enable
If BPDU frame come on this port, port will be disabled preventing possible switching loop.
Switch# show spanning-tree ? Switch# show spanning-tree interface fastethernet 0/1
1. Lowest bridge ID (Priority:MAC Address) switch becomes the Root-Bridge
2. Each non-root bridge should have ONE root port (RP) which is the port having lowest path-cost to Root Bridge.
If the path cost equal, then here are the criteria:
3. All ports in Root Bridge become Designated Ports (DP)
4. Each segment should have one Designated Port (DP)
5. All RP/DPs will be in FORWARDING state & all other ports will be in BLOCKING state.
You may wish to look at this animation:
You can see some detail here also: